I don't know what heli so I'm going to guess it's the HBK2.
There are a lot of things it could be.. or a combination of things..
A couple quick, easy checks:
If you replaced the blades and they are wood, check to see that the blades are installed correctly and that they aren't warped.
not responding well to the sticks
Check to see if feathering shaft is even and isn't tightened too much. The blade grips should turn easily and smoothly when the linkages aren't connected.
A quick check to see if the flybar is straight and that both paddles are level.. then check the tension for both blades in the blade grips so they are even and loose enough.
If those things don't help ya at all, then you can move on to other parts being bent like tail shaft, main shaft or alignment issues like gear mesh right on down to bearings etc.