Why wont my car stay aligned in the rear ? on 2000 Hyundai Accent

I have had a 4 tire alignment 3 times in 7 months and the rear is out of alignment again after 2 months , I replaced the rear wheel bearing and 2 rear tires + 4wheel alignment for $500 and they said that should fix it but it didnt. Now its up for inspection before they take me to the cleaners does anyone know what can still be causing this?

Asked by for the 2000 Hyundai Accent
i hate to say it but i think your unibody or subframe is broken in the rear.i've only seen this twice but it does happen and there's no way to align the wheels if theres a crack in the frame.only momentarily can it be aligned.
1 more answer
Back up a second - what is physically happening to the tires that is indicating an alignment issue? And, are you getting a print out when your alignment is performed showing before and after specifications and measurements?