Lower right back teeth ache but my dentist can not find anything wrong. Any thoughts?

Endodontist. Endodontists are experts in diagnosing toothaches. Many cracks cannot be seen either visually or on x- ray. Endodontists who work microscopically can find and diagnose much better.
Maybe cracked. Your tooth may be cracked. It's hard to see that on an x-ray, and even sometimes with visual inspection. If the pain doesn't stop, ask your dentist to check for a cracked tooth with a light. When shined on a cracked tooth, we can see where the light shining through gets blocked.
Bite. The first place i would look is at your bite. Next gum health. Try seeing a dentist with pankey or dawson bite equilabratiom training.
Second opinion . If your tooth is really hurting, get a second dental opinion. Could be your wisdom teeth.