OAuth 2 refresh token invalid_grant

Hello,

I am attempting to implement refresh tokens but my refresh request always ends up as HTTP 400 invalid_grant. This is a webapi project using OWIN and OAUTH2.

Any ideas what might be the cause?

/// <summary>
        /// Creates a refresh token
        /// </summary>
        /// <param name="context">
        /// The authentication context
        /// </param>
        /// <returns>
        /// The new refresh token
        /// </returns>
        /// <exception cref="OverflowException">
        /// The dictionary already contains a refresh token ticket.
        /// </exception>
        public async Task CreateAsync(AuthenticationTokenCreateContext context)
        {
            using (var ctx = new MpsDbContext())
            {
                var refreshToken = new RefreshToken
                {
                    RefreshTokenId = Guid.NewGuid(),
                    Expires = DateTime.UtcNow.AddMinutes(5),
                    Issued = DateTime.UtcNow,
                    ProtectedTicket = context.SerializeTicket(),
                    Subject = context.Ticket.Identity.Name
                };

                context.Ticket.Properties.IssuedUtc = refreshToken.Issued;
                context.Ticket.Properties.ExpiresUtc = refreshToken.Expires;
                ctx.RefreshTokens.Add(refreshToken);
                await ctx.SaveChangesAsync();
                context.SetToken(refreshToken.RefreshTokenId.ToString());
            }
        }

        /// <summary>
        /// Validates a refresh token
        /// </summary>
        /// <param name="context">
        /// The authentication context
        /// </param>
        /// <returns>
        /// Nothing is returned
        /// </returns>
        public async Task ReceiveAsync(AuthenticationTokenReceiveContext context)
        {
            using (var ctx = new MpsDbContext())
            {
                var clientRefreshToken = Guid.Parse(context.Token);
                var result = from n in ctx.RefreshTokens where n.RefreshTokenId.Equals(clientRefreshToken) select n;
                var refreshToken = result.First();

                if (refreshToken != null)
                {
                    context.DeserializeTicket(refreshToken.ProtectedTicket);
                    ctx.RefreshTokens.Remove(refreshToken);
                    await ctx.SaveChangesAsync();
                }
            }
        }

        /// <summary>
        /// Create a refresh token
        /// </summary>
        /// <param name="context">
        /// The request context.
        /// </param>
        /// <exception cref="NotImplementedException">
        /// Synchronous operation not allowed
        /// </exception>
        public void Create(AuthenticationTokenCreateContext context)
        {
            throw new NotImplementedException();
        }

        /// <summary>
        /// Validate a refresh token
        /// </summary>
        /// <param name="context">
        /// The request context.
        /// </param>
        /// <exception cref="NotImplementedException">
        /// Synchronous operation not allowed
        /// </exception>
        public void Receive(AuthenticationTokenReceiveContext context)
        {
            throw new NotImplementedException();
        }

 public override async Task GrantRefreshToken(OAuthGrantRefreshTokenContext context)
        {
            var newId = new ClaimsIdentity(context.Ticket.Identity);

            var newTicket = new AuthenticationTicket(newId, context.Ticket.Properties);
            context.Validated(newTicket);
        }

refreshToken = function () {
    var postData =
        {"grant_type": "refresh_token","refresh_token": sessionStorage.getItem("MpsRefreshToken")
        };$.ajax(
            {
                url: "token",
                type: "POST",
                crossDomain: false,
                data: postData,
                headers: {"Authorization": sessionStorage.getItem("MpsAuthToken")
                },
                success: function (data, textStatus, jqXHR) {
                    sessionStorage.clear();
                    sessionStorage.setItem("MpsAuthToken", "Bearer " + data.access_token);
                    sessionStorage.setItem("MpsRefreshToken", data.refresh_token);
                    sessionStorage.setItem("MpsUserName", data.userName);
                    sessionStorage.setItem("MpsUserId", data.userId);
                    sessionStorage.setItem("MpsUserEmail", data.userEmail);
                    sessionStorage.setItem("TokenExpires", data[".expires"]);
                    sessionStorage.setItem("TokenLifeSpan", data.expires_in);
                    tokenLifeSpan = parseInt(sessionStorage.getItem("TokenLifeSpan"));
                    timer = window.setInterval(checkTokenExpired, 30000);
                },
                error: function (jqXHR, textStatus, errorThrown) {
                    window.alert("error");
                }
            });
}

POST http://localhost/mps/token HTTP/1.1
Accept: */*
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
Authorization: Bearer hNlwBHqUbKYPRz41rriwGKG9x6G5MD-uCWX5ftYTM8P-8MeQ7qXB7xtn0KeIVKBP3jUDMQVenB4LgBMOkrq1Cn9bkt_jIqQv-rUEr8eyJ67kxxQ5kHKMbzcsjl4jgeyIEJxQe9efhFf2LoZ9pPIdbfkKfKG4BUUMpvRmHHbzju-X20Ok9CzEDSE-KPKv2i9vpsuI9391uprHFkluzP-58jL8GNH3al7KkgYRCb9syANsCq6MY1s5Hhj0UrYwIOPP5tNs2MO0wNVOg86uH1qxBl1zof24aF1TQzJ6KCRHKyxFmyUj3aEd0HTAsyvHAUx-KAi7miwMyzfzkSc7nodlYwSeXPLd2FMQffQMQwUOdGZo_ol03cib294--yg5WOWCGEqWCIHcrl6WauJ9_e4MGudD3VzGqmV1VeuPJV9lHGZ-KiyepnVVbd0IemMCTQBg5dbNgaUQa6Hab3IWev_J9O9gs01UrhMuqTfyuNZ6wv7S1_477MMC_OfkKLqykS6ZMRhqoOzyb2afMwWbxM7pYCpxHO20-8ys9sKCvhVQLFW4LnPdmYvuaMgJMLWxNUS2oNggh-Ro3jLAYfxiElrjNN0fvDK7ouOO16wv-bxgSzoVwCwPdK0XW6Ejj9pjc-QoJb0HBnsXHyURyYxEgK0KTPesCCs
X-Requested-With: XMLHttpRequest
Referer: http://localhost/mps/default.htm
Accept-Language: en-GB,en;q=0.5
Accept-Encoding: gzip, deflate
User-Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; Trident/7.0; rv:11.0) like Gecko
Connection: Keep-Alive
Content-Length: 75
DNT: 1
Host: localhost
Pragma: no-cache

grant_type=refresh_token&refresh_token=03f325d3-3635-e511-8266-4c8d79e0337d