Once you rule out any structural or joint/unsoundness issues and deduce it to a strength issue, there are exercises to increase coordination.
I have a large gelding with a fairly long body, so sometimes it takes a lot of conditioning to connect the front to the back. He came to me green with years of being left in a field, so it took and still takes a lot of conditioning in walk/trot. The first step for me and my gelding was to build up his back muscles well enough for him to carry me, and our fitness. It is also important to note that if I wasn't balanced - neither was he. I found long and low exercises useful for engaging his back and hind. It took a lot of transitions (halt, walk and trot) to move into the canter. When my trainer and I introduced the canter there were half halts to balance him, also spirals and twenty meter circles helped. But the most beneficial exercise (or the one that made the most noticeable impact) for getting his hind end under him was halt to canter transitions. It's just what worked for him and myself. Your coach will offer you exercises that work for you, I'm sure. Good luck;)