This is a back to basics question that most of us overlook after a long time of practice. Try changing the needle on the syringe after drawing up the medication. The handling of the needle in vials and rubber stoppers tend to dull the needle and is more traumatic to the tissue. You can actually feel the difference in resistance. Also is it possible that some members of the team are "hubbing" - applying too much pressure and causing the hub of the syringe to bruise the skin?