Pro Lite trigger sticking

Sounds like there's a broken trigger return spring. The reason it will re-set when the bolt is pulled back is that pulling the bolt back pushes forward on the sear which pushes up against the trigger forcing it back forward.

They can be found here: Link

I'd suggest ordering a couple, as well as any other wearable parts you can find on that page since Tippmann discontinued pro-lite parts sometime back in the early 2000's and most people ran out of stuff by 2009. They're great markers and real tanks, but they won't last forever. That said, be careful when disassembling the lower receiver (grip) as you don't want to lose or damage anything. It's not difficult, just use care and snap pictures of each step of disassembly so that you know how everything should look as it goes back together. Also, when you remove the grip, make sure that you DO NOT lose the little metal square/rectangle that is held in place by the forward receiver thread bushing. That's the valve stop and the marker won't work right without it. If all else fails and you can't get it back together right, take a look at this schematic:



Hope that helps, and good luck!