A/C does not always get cold?

Hello, Started about two weeks ago. The a/c runs but does not get cold. Other times it gets cold. I can hear the differance in the motor (park) when I turn the a/c switch on.

Any Ideas? Why the a/c gets cold and other times when running it blow regular air with the a/c switch on?

I just check and recharge the A/C with a kit from Wal-mart. The gauge stays in the green........Aloha