A friend of mine's cat had htat problem too. The vet said it was due to the fact that she was living rough for about 6 months - so without good food Jasmine's teeth didn't fall out. She had them surgically removed and is much better for it. I don't know how common it is but I think animal teeth are just like human ones - if they baby's don't fall out of their own accord you need to have them removed.