hi sarah!
even i m facing the same prob in my hemi patients.
with them i have found that they get proper reduction of shoulder only when the muscle is contracting but as soon as the tone is zero i e if the deltoid relaxes the humeral head will be out again. but only thing which is to be taken care is to prevent further lengthening of joint capsule, ligaments and muscles arround. Use isometric holding i e hold relax and rhythmic stabilisation beginning from mid ranges to extremes of ranges. Also try manual resistance or weights to make use of gamma bias. All the very best!
Moiz Bohra