asking for rein back increases the need for the horse to think about where the hind legs are, lift the back etc. Sask for three/five steps back then continue forward again.
It's a case of having lots of different things to do, keeping the variety but also the pressure for the horse to keep trying.
Use a light seat on the hill work for starters- up and down. walk and canter work better for strenthening the back muscles.
Also, try asking the leg forward with your seat bones, get into a rythum and ride with these bones, that should help focus the horses feet.