I have gone 1.5 in delts many times and so have friends,with no trouble.However many feel it's a problem for a smaller delt,all these boys were well over 200.Slight chance you may have injected between the fascia (anterior and medial delt seam)was your injection straight in or did you accidentally angle the dart a bit forward.Massage the area.Have you hit delts alot,possible scar tissue hindering dispersion.