A number of things could possibly be causing that problem. You will probably need to provide more info on the symptoms to get any kind of solution.

Pitch angle - if it flew before and you haven't changed anything this shouldn't be your source. If you crashed it and rebuilt, and now it won't lift off but the engine makes good rpm and the blades spin then you could have assembled something off so your pitch range has changed.

Motor - Could be running too rich, not making enough power, but you said it sounds perfect. Does sounding perfect mean that you can advance the throttle and it runs up to high rpm without a bunch of hesitation etc? If that is the case, it could be your clutch. Possible that it is slipping and failing to transfer the needed power to the blades.

You could check rpm with a tach, check your pitch range with a pitch gauge. Maybe your fuel is old and your engine tired, the combination of the two isn't giving you enough power. Could probably be a lot of things, you need to get more descriptive on what you find and post the results. I don't have much experience with nitro helis (I'm on my first one) but from cars etc, usually when the clutch is failing it seems to engage at higher rpms and slips a good bit.

It is the only real item between the motor and the blades though, so unless you have got something going on with a large amount of drag in the system (robbing power) and the motor runs fine / pitch checks out, that would be where I would look.