You have a few possibilities on what is wrong:
1) Drive spring is switched with the cocking spring... switch em back
2) You have a small leak or series of small leaks from your fittings and/or within the gun.
3) Something could be funky with that new power tube of yours (switch it with the old one, see how you do then)
4) Your flatline is not installed properly
5) Your reg may have a defect
6) Your tank maybe screwed to far into the ASA (if it is screwed in to tight it limits air flow)
My advice, do a water test, teflon tape ALL your fittings and make sure they are tight, switch that new power tube with the old one, crank your reg and try a new tank or the gun without your reg with a new tank.
Paintball is a sport that is it's own worst enemy. From gun owners ruining the average "weekend warrior" with a Shocker to some yahoo shooting up his neighbours house with a BE Talon. If you want paintball to not be limited by bad reputation and government, then speak up for YOUR sport or You may not have one oneday.