Really push her/him forwards and INTO the bridle.
Since you have no access to hillwork you'll need some exercises to help you to ask her to engage her hind end.
Transitions are great for this. Including transitions within the pace, so reallly slow her down in trot to ALMOST walking, hold for a few strides, then push out again to a normal working trot, then again into a even more forwards, more medium trot, then back down again. She'll have to use her hindquarters to push herself around.
Leg yeilding in and out of 20m circle, yeild in to a 10m circle then back out to a 20m. It'll work individual muslces and if she's going forwards correctly and she's correctly bend through your hands and legs, her hindquarters will have to be engaged.
I'm not sure what your exactly working on with her but Shoulder ins also bring the horses back onto their hindquarters