It's more than just using it correctly. My DD (who is 15) was born during the "rage" of ear thermometers!

We always used one on her and I always thought it was pretty accurate.
Then my son came along. For some reason, the the ear thermometer never registered well on him. He could be RAGING with a fever and we would get something like 97.9! Even at the doctor's office. They were always looking at me suspiciously even though he was burning up.
Turns out that he has very narrow ear canal and always has a lot of wax in his ears (he also has a lot of ear problems and ear sensitivity

). Anyway, no matter how you hold the thing, it just does not work on him. I'm sure he is a rare case, but I have spoken to other moms who report the same thing.