how do.
how long has the pt had this problem?
hows the pts scapular mobility and strength of scapular stabilisers?
whats scapulohumeral rhythm like with active flexion?
is passive range of abduction limited as well as active?
if its stiffness thats limiting abduction i find maitland longitudinal mobs just before the point of max abduction tend to help if done properly. important to actually remember the orientation of the joint though when the arm is abducted and make sure its an inferior glide thats happening.