I keep looking at these pictures, and I just don't think she's ready to "engage the hind end more". JDI, please don't take offense - she looks wonderful.
Speed and "pushing them into a frame" just isn't how dressage works. I fully agree with what JDI is doing in bringing her along the way she is...building muscle as she goes.
My eye says she actually IS using her hind end quite well; especially for only having been in JDI's care for 4 months. :) The really powerful work will come when she begins to collect her; the head comes up and the rump goes under :)
Yay for Maia!