It is a little hard to know exactly what is the problem from your description. My first guess would be that the joists and studs were not "crowned" correctly. Typically, most framing lumber, when viewed along it's length, on edge will show that it is bowed in one direction, i.e. not perfectly straight. This is normal to a certain degree. If this is what is the problem then you can probably fix the joist by just flipping it over. Ideally, for joists you put the crown up so that the weight of the floor and whatever is put on the floor will tend to flatten it out. This is how "pre-stressed" beams work, to a degree. You should try to have a carpenter look at it, as a decent one would know what to do in about a minute. A bad carpenter would just stand there and scratch their head.