I agree with Ethan, however, I would not concern myself one bit with collection, without having good forward impulsion first.
I am going to make a wild guess that she may have been trained WP, and has had it been drilled into her head, "trot slow", and this by having the rein snapped every time she raised her head a little, and thus the rider assumed she/he was "collecting" her. Actually, such activity was only dampening down the energy that the rider would require for collection. She has then developed the way of going where all her energy is dampened down to the point where she has no suspension at all during the trot, and it becomes a "shuffle".
I would completely give her her head and ask her to trot out. If she canters, bring her back to a trot, but then if she dies out , ask for more. Keep asking for more, but only shtting her down if she canters. Eventually , she'll figure out you want a good fast trot. Post the trot, if you can, as this will encourage a bigger trot from her.
Once you get more forward, you might get a bit more contact on the bit and move her hind end under her and toward the bridle (collection), but not until she is honestly forward.