plastisol screen print faded significantly after 1st wash

Was it water base or plastisol prints?

If it was water base, this might be normal depending on the print and circumstances as it does fade a little. If it was plastisol then it is most likely the result of improper curing. If it was water base and it faded a lot improper curing could also be the problem.

If the printer does not have a conveyor dryer and is curing solely under a flash that would explain the issue.

You should definitely ask the printer to cure them again, that is if they have a conveyor dryer. They could easily send them through the conveyor dryer after the fact and cure them, but you don't want to give the rest of the garments to your customer until you're sure they're cured. I would also say that if the printer does not have a conveyor dryer and only a flash, then don't send them there to be cured... Another shop in your area may be willing to help you if it means they may get your future business.

You probably don't want to heat press them, although it would work. If you do, use some dazzle trans on the press. This will give it a really glossy appearance, but most importantly it will prevent the ink from sticking to the top heat pad on the heat press. Do be aware that the heat pressed shirts will look significantly different than the screen printed shirts that were not heat pressed.

First, the printer should rectify the issue for you since it is their mistake. If they won't or you don't feel confident that they can, the second best route I think would be to try another printer and see if they will cure them for you (I would expect to pay a minimal charge for this). Otherwise, yeah heat press them. Dazzle is similar to the teflon sheet, and the teflon sheet should work just fine. Dazzle will make it glossy and it should not leave any film. The reason that it looks so glossy is that it is a super fine super smooth surface. Teflon sheet could make it glossy depending on the sheet, or it could make it look textured like the sheet of teflon. Try one and see how it turns out. Better yet, try one, wash it, and see how it turns out. Now, depending on the ink deposit, it could look different when you're done. If it is a light ink deposit then it should be just fine. If the ink deposit is heavy, if it has been built up with a thick underbase and there is fine detail, you may lose some of that detail since the heat transfer press will in essence squash the ink a bit and close areas with fine detail, maybe. The proof will be in your test press!