On a serious note - had a similar problem with the current mistress. She couldn't get her mouth open wide enough or keep it open long enough. I'm quite small, cockwise, especially with thickness. But I still felt her teeeth and she insisted that she was trying.
So, I showed her a video of a good BJ. After the initial whining (am I no good? don't you love me then?) she admitted it was useful and improved slightly.
A few months ago she went to the doctor (non-related reason) who remarked that her jaw didn't separate properly and asked if it caused her problems. So, long story short, we found the best position and angle is 69. She can get her mouth round it properly most of the time now.
and she doesn't need to visit the dentist anymore.