New filling keeps coming out

[dragon53]

I had two metal fillings about 50 years old that came out about the same time. My current dentist put in new fillings. But one of them (right, lower, rear molar) keeps coming out within a day or so after being put in----it has come out three times.
My dentist says the filling is in a position similar to hanging on a ledge and makes it easy to come out---but the original metal filling stayed in for about 50 years. The dentist said a crown might be the best answer, but she wanted to try the filling approach first.
So is there such a thing as a tooth that can't be filled (even though the original metal filling lasted 50 years) or should I see another dentist?

 

[MattKW]

It would be interesting to know why the 50yo filling failed; often it's when part of the natural tooth breaks away and the filling loses grip.
Is this dentist trying another amalgam, or a composite?
Any Xrays to show us?

 

[dragon53]

MattKW:

I had two 50 year-old metal fillings, applied at the same time, come out within a few weeks of each other. The new dentist told me I'm grinding my teeth which is wearing the tooth down, so maybe that makes it difficult for the new fillings to stay bonded. As I posted earlier, a crown is the other alternative.
The dentist did two new fillings on other teeth, and they are fine. I just had the problem losing three new fillings on the same tooth.
I'm curious if losing three new fillings is normal under the circumstances.

 

[MattKW]

So it's a bonded composite? More likely to fail in a high-stress situation like a back molar. It may be that there isn't enough thickness to do either a composite or amalgam. For 2nd molars, I often go for a thin precious (high-gold) crown.