Martin is right your horse may not have enough strength in his/her top line and hindquarters. Some things that can help will be to ride long and low letting your horse to stretch.
Another thing that is good is to work them over trot polls and slowly decrease the distance in between them. This will make the horse work and really used their buttock muscles. One key thing to look for while doing these exercises though is to make sure you are riding the horse through and forward
Hope that helps