Transitions from one pace to the other should help, e.g. trot, walk for a couple of steps, up to trot again, try to repeat as many as 4 times in a 20m circle, or trot six strides, canter six strides. Also, transitions within the pace, going from working trot to medium trot and back again, the same with canter.
You can also try gradually decreasing a 20m circle to 10m but keeping the inside leg on to encourage the hind legs to come under, then gradualy leg yield out, but try one or two steps of leg yield then back onto that circle to re-establish the engagement, then one or two steps of leg yield.
I saw on a website (https://farriery.com/2012/03/17/horse-is-standing-under-himself/) many explications.
I hope this helps, bye,