You’re not the only one to have this problem, from the post below, but unfortunately I don’t see a solution. It might have to do with frame skips, or that the signal is sent for too short a time. There’s a “sync timing to screen refresh” option in the parallel port component, you could see if checking that helps.
The only way I can think of testing each individual pin is to make a dummy experiment that tries sending a signal from each pin in sequence.