Sounds like you may have a spring problem. The spring is located under the barrel shroud. To remove the shroud remove the three small screw that hold the shroud to the barrel retainer(the larger part that has the 3 pins) they may be under the foam grip. If your spring has a collar with 3 set screws you can adjust the tension by moving the collar. BE VERY CAREFUL tightening the collar screws its easy to dent the barrel if you over tighten the screws. Also nova barrels like lots of lube on the o-rings at the back of the barrel. Here's a link with nova breakdowns and more info.
ZDS Paintball - Marker Tech - AirStar Nova