Sadly many doctors and nurses will not answer a call for help as they are far too worried about possible litigation. This in spite of the fact that nurses are officially required to help in any emergency up to the level of their current experience / training.
The possibility of litigation is a very real one but one which can be eased by airlines agreeing to idemnify any qualified person willing to help. Crew should also remember that not every passenger who has the title "Dr" is necessarily a medical practitioner. There is an excellent InFlight Nursing Course which I am hoping to do this year - last year's course was cancelled at the request of the CAA - for security reasons. It isn't only within the aircraft itself that help may be needed. I once received a VERY welcome upgrade from BA becuase I bothered to spend five minutes trying to ascertain the nature / severity of the illness suffered by a passenger on the airbridge at Singapore. BA's attitude was "If you help us, we will help you" I had a blissfully comfortable trip back to the UK as a result and received really special treatment from both ground crew and cabin crew.