Lame in walk virtually sound in trot

[Marie231]

I have a very annoying issue and after serious investigation with vet and physio everyone is scratching their heads. My mare is very short on her right fore in walk, no nodding. It's on a circle on the lunge under saddle and in a straight line. She's sound in trot on the lunge in a straight line but slightly short under saddle more noticeably on a contact.
The physio couldn't find anything tight, sore etc at first. Initially the mare was given 6 weeks off, no improvement.
Then the vet nerve blocked up to the knee and no difference.
The vet then scanned and x rayed neck, shoulders, withers and back but nothing visible!
The horse was put into serious work and it gets slightly better with work but never right.
The physio came back a lot and at first said she thought there was nothing wrong but then her right poll was out and after a month her left poll was out but no other soreness anywhere.
Does anyone have any suggestions?
We are looking at sintigraphy but my vets worried it won't show anything and the mares not insured!

 

[ycbm]

Ooh, this one's a challenge!

I'd be looking for something outside the box. I had a horse who was lame in front from a sacroiliac strain that fooled us for weeks. So I'd look on the back end for something that is compensated for in a symmetric gait but not in walk. Or maybe deep shoulder soft tissue injury - have they bent the leg and pulled it right out sideways?