What are some things that can make a horse sore in the loin other than poor saddle fit?

Hocks/stifles

anything else?  they've been vetted

All I can tell you is hocks, especially, very rarely show up in a flexion. Sometimes stifles don’t either, or it’s a very slight positive or maybe just change in dish of the stride.

if you rune your finger down the semitendinosus muscle that is parallel to their tail, with moderate pressure, and get a reaction they are definitely sore in hocks/stifles.

Did your vet diagnose them sore in the loin?

No. It happens when I palpate the area

I’ll definitely try that! Thank you

I will be too, as mine passed flexion too. Not try to steal, but definitely interested!

saddle fit/wrong saddle pad, hollowing out their back, wrong angles in hooves/too much heel.

My gelding always palpated sore over his loins. He was a very sensitive horse and he ended up having stifle issues. Took the 4^th vet’s opinions before they found the bone spur and damaged ligament. Some other ideas that could cause pain: sore hocks, underrun heels, saddle fit, kidney issues?

PSSM can also cause soreness there.  

We have one with kissing spines between T15-L3 gets sore there when he’s due to be injected.

He flexed 100% sound when we were trying to diagnose him.  Because of the soreness we decided to do an X-ray - thought we were maybe looking for something SI related.  Nope.