It is quite possible that they didn't hear you - modern cars can be very quiet. I am slightly hard of hearing and other riders often tell me that a car is behind before I realise it is there.
I was recently in Scotland with a friend who is almost deaf in her offside ear. She had a car creep up behind her on a hill on a singletrack road and didn't hear it, nor did she hear me calling out to her to pull into a passing place. (I have suggested that she should consider getting a mirror, or at least looking behind more frequently. The same applies to me.)
I think the best solution would be a very short beep on the horn, though a few riders will react badly to that, misinterpreting it as aggression.