3yr old with a short stride

About 6 months ago my 3 yr old came up with a short stride. Just out of the blue nothing changed it was just there one day when I rode her, worked out after a bit and then the next day it was there the whole time. What's weird about it is that is only shows up when you do a slower jog type gait and when you speed up it goes away. She does not bob her head or give any other indications that she is in pain. We had the vet come but the results of blocking were inconclusive. Has anyone else come accross this before?

As horses balance and use themselves differently in different gaits, there can be pain at the trot but not otherwise. Could be pain in her shoulder or other joints, but is also likely to be related to hoof balance, breakover and how she lands. Of course can only guess with this much info. Perhaps at the trot she's landing more toe-first which is putting extra pressure on the laminae of a long toe or something. Pictures would give a better idea.

I'd be inclined to find a good equine bodyworker who is knowledgeable about hoofcare and put her thru her paces in front of them.

Sometimes a horse that does a slower gait will take smaller steps instead of just moving slower (if you kno wwhat I mean)....so it might just be she does that also.
My horse has a shorter stride when I ask him to do a slow trot, but when I ask him to extend he stretches his legs and extends nicely

Short-strided where? In the front, back, all over?

It's just in her front left

Would you be able to post a video?

Hard to know by your post if it's a medical/traumatic injury or pain that is causing the short strides or if its simply lack of engagement and forward movement. If you can post a video it would help trying to figure out what is going on.

Since it's been mentioned that it could be a trimming/shoeing issue, I wll skip that part.

A three year old is still developing and you say it's been 6 months since he started being a little off, so I'm going to assume you've been riding him since he was a 2 yr old. You might try slowing down on your training and consider letting him rest until next spring. Whatever is going on, he's still growing and this could be your first sign his body isn't ready for the workload just yet. Better to back off now and have a sound horse for life than to push for a competiton this year and he's retired before he's five.