Game Center completing achievement not working

I am testing a development IOS build using the sandbox mode for Game Center. I am able to log in to my test account just fine, but when I report a completed achievement it is not successful. Here is the code I am using to call the achievement:

Social.ReportProgress("achievement ID", 100.0f, (bool success) => {

				Debug.Log("achievement success: " + success);
			});

Is there something else I was supposed to do before calling this?

Make sure you call Social.Authenticate first. Also, I believe there is a bug with Social.ReportProgress on iOS, so that may be your problem also. I have this same problem and I had to use a third party plugin to get around this.

Unity has a bug with unlocking ios achievements. It’s been here for a while but hasn’t been fixed yet.

A solution was posted here by the Kind, the Smart, the Only : Marcpeyre.

The fix worked for many souls, including myself.