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.Ne ...