In this article about training simulators delivered to the Air Force base that will train all F/A-22 pilots, there’s this snippet:
Initially, the training will be only for air-to-air operations. That is because the original F-22 (before it was renamed F/A-22) was not designed as a primary ground attack platform, but as an air superiority fighter. “The current F/A-22 avionics software (the operational flight program) does not support air-to-ground,” said Riegsecker. “This will be part of the program in the near future.”
So basically, even the simulators aren’t set up to perform the ‘A’ part of the F/A-22’s mission.
My guess is that they’re having trouble reprogramming a system designed to simulate a cutting-edge supersonic, stealthy air superiority fighter to make it also act like a slow, heavy, flying tank. All at the same time. But however hard that task might be, I’m betting that it’s a lot easier than making the actual aircraft perform both tasks.