Go back to slow work. Backing up, rollbacks down the fence, ground poles, and hill works are good for getting a horse to use their hind end. Also go back to slow work around the barrels. Walk or trot the pattern and ask your horse to collect himself before the turn. Also when your walking the pattern you can stop right where you want your horse to start rating and ask him to back up to get him on his hindquarters.
Take the horse completely off barrels and do a lot of slow work. Rolls backs, back up. The horse needs to learn to use its entire back end as well as pick up it's shoulder before going on barrels.
The best thing I can tell you is do hill drills and work ok roll backs and set up cones around the barrel that can help them be more aware of where there body's are