How are you asking her to trot?
Ok, how is she off the pattern then? Is she light off your leg moving forwards? Can you move her hindquarters, ribs and shoulders independently? Does she move off your seat? Is she soft mouthed, does she have good lateral flexion, vertical flexion, a good back up, stop etc? Are her leads solid? Those are a few basic things that should be solid before attempting to pattern her. If you have any holes there, they'll be very visable once you start something else with her.
Edit: Is this the same mare that doesn't tie? If so, that can speak volumes to a few holes in her training right there. It could also mean at some point when she's pulled back that she injured herself.