Sounds like your problem isn’t with Psychopy, but connecting with your scanner. We use a Cedrus Lumina box to receive triggers to a Siemens Prisma. Are you interfacing the Labjack directly with a scanner trigger cable or running through a controller of some kind?
To be clear, whether using the Cedrus Lumina controller or others, it is typically the fMRI scanner that is sending the TR or slice triggers. The scanner does not receive them.
I am not familiar with the Labjack T7-Pro but something to look for is pulse duration. For Siemens, the trigger pulse duration is typically in the 20µs range – very short. GE scanner triggers have longer durations. The older GE models had a duration of 500ms but I’m not sure what they are on newer ones.