i`ve run into this alot. usually i can fix it by using longer screws and shimming the tabs on the gfi to the current wall depth. another thing you might watch for is nails that run all the way through the box. these won`t give you enough room to put a gfi in. you might consider a gfi breaker at this point. sometimes i`ve had to take the old box out (not always easy to do) and install a remodel box. thinking it out ahead of time really helps. good luck.