The observation that it sometimes works and sometimes doesn’t suggests to me that you may be experiencing the same issue I did, where a particular bit of the parallel port that controls whether it’s reading or sending data can get flipped. When flipped, it’s set to “output” mode and cannot detect the trigger input.
See my post here for a write up on what could be going on. In the PsychoPy terminal, you can try running the test code lines to flip the control bit.