What do you mean, "should work" - You don't sound too certain!

How have you tested it?


A few favourites:

Timing - the simulator knows nothing of the timing requirements of your hardware.

Configuration: If your configuration of your simulator doesn't match your actual hardware

Initialisation: If your code relies upon the way your simulator initialises (eg, clearing all memory to zero), but your real hardware differs

Wrong Simulation - the way you are simulating your hardware does not match its actual behaviour

Hardware misbehaviour - whether due to faults or design flaws, or both!

any combination of the above!