Resistant to local anaesthetic (dental)?

Does anyone else have this problem? I have known for some time that the freezing used when I get a tooth filled does not always completely numb the area. I had to have a root c, and because of this freezing problem, the dentist gave me the highest dose she could, which involved one and a half shots to the outside of my upper gums and a shot on the roof of my mouth (which the dentist said normally isn't done because it hurts, and when your mouth goes numb so does your throat, which feels like you're choking). Even with these extra measures, the freezing wore off completely and without warning in the middle of my root c (about 40 minutes in, just as the dentist was fishing around high up inside my tooth for the last little bit of the root - she gave me more and had to wait for me to finish crying). How common is this problem? The freezing takes, but it just doesn't last.