The worst locking is when he’s been resting a bit and then goes to move. It’s not just sticky, it totally locks and drags behind him. He seems annoyed sometimes but honestly I’m grateful cause he’s a pretty easy going guy who doesn’t get stressed by much. I’ve taken ridden work slow with this horse, really tried to build strength from the ground first, but even with doing poles/hills/lots of walking, there isn’t improvement. He doesn’t typically lock when I’m riding, but that’s mostly because I always handwalk for a quite a while before getting on. Circles of any kind, lunging or ridden, make him lock more the next day. I haven’t jumped him at all, but he’ll jump himself over logs in the field. We have x-rayed the stifles (no OCD that we could see).
Perhaps back him up first before asking him to move forward after the resting period. It might ward off the locking. But definitely take a closer look at those feet!
I’ve got a growthy youngster who periodically goes through periods prone to stifle locking especially when his hind legs get relatively too long for his body. He’s on a 4-5 week shoeing cycle and his angles are fine after a trim (have taken X-rays) but by the end of the cycle he just seems to get too tall in the toe relative to heel growth and then will be more likely to lock. Had my vet out for a routine check one day and they were a bit alarmed about his stifles. I said, well if you can wait a couple hours (they had other horses to see), he is getting his feet done today…Take a look at him after? They repeated the same tests (jogging as well as walking up and down a steep hill) and declared him to be totally normal once he got trimmed.
this isn’t about the stifle issue, this is about balanced trimming 
That said, a good farrier SHOULD know how to tweak the hoof angle to ease stress on the stifle
Sometimes it’s a growth spurt that puts them butt-high with a straighter stifle which causes the locking. When they level out, it’s gone. And yes, improper trimming can exacerbate the issue during that time
