Før people with declining hearing (such as myself), sometimes it is the pitch of the voice. I have trouble understanding some people who speak standard English at a reasonable rate of speed. If their vocal register is just wrong I just can't hear the information-rich part of what they are saying. If I can't see their lips, or if there is any background noise or conversation, I just can't understand them. I, too, frequently just nod and say 'mm-hmm' randomly, but NOT when it concerns my health or anything else important. If you are having trouble understanding a health-giver, don't let them leave until you do understand.
In the case of a nurse with an accent, it never hurts to be a bit flattering. "It makes me feel bad when I don't understand you; you take such good care of me." (After all, it's the nurses who really run things.)