Yup the NR can take a while before it gets to it's max power. In the NR91 owners thread Stuart mentioned he'd gone through 18 tanks before the engine opened up. Also, be sure that you have your carb on very tight. Luca recommends you push the carb on as tight as possible before tightening the clamp. If the engine's already in the heli, loosen the clamp, and use a medium driver or a main shaft as a lever to push the carb down from the fan shroud and tighten the clamp screw. This is because the inner O-ring, not the outer one on the carb, needs to be compressed very tightly.
See my artwork: http://martins819.deviantart.com