What POS software are you using?
Is the printer running as a windows printer or are the drivers built into the software?
If you are using a printer driven cash drawer that is a Windows printer look in the driver pack and se if there are settings to allow tone alerts when the printer prints. If so you could turn that on to test the drawer as typically the sentinal that is used to tell the printer to beep is the same sentinal used to tell the drawer to fire.
That said, if the drawer opens sometimes but not others test the same function that gets it to fire once and then try to narrow down when it works and when it does not. (move cables, restart printer etc.)
The problem could be printer, drawer, cable, software, com ports....