I'm just trying to get a better idea so others can jump in. She may not be getting enough exercise to build her rear end. What else does she do besides walk? Is she off leash to play? what does she look like when she runs, turns, etc. Is she long legged? A little more info might be helpful.
'If you find a way to enjoy the ride, it doesn't really matter where you end up'
5 GSDs at the gate: Katie, Iroc, Red, River, Jake