[Help] Used marker not firing properly. TPN Sierra One.

Help guys, I recently bought a used TPN Sierra One off of Kijiji and it is in immaculate condition. I have found one flaw recently while testing it out. It seems as though when the bolt is cocked back, there is an excessive amount of trigger pull needed to release the bolt and even then, there have been times where I can pull the trigger physically all the way back and the bolt doesn't even release. Other times it works just like it should and it releases on the slightest trigger pull.

Im going to take the gun apart and reassemble it tonight, maybe when it was previously cleaned the trigger wasn't put in correctly? or maybe even the trigger is physically damaged not allowing it to release the bolt when it should?

Any advice would be greatly appreciated as I should be going on a paintball excursion this Friday and I picked up the gun so I could have a friend come along with me and save a little money while doing so.