Tips for horses that don't respond to leg??
Once you determine that there is nothing physicallly wrong, I would start using a dressage whip just to aid with keeping the horse in front of your leg. Don't use a spur until your horse is more responsive to your leg. Hope this helps :))
Do you have suggestion to build horse's strength?
In the ring, I usually do lots of trot to canter transitions and pole work. For jumping, build a gymnastics line. Also if your able to go out of the ring, lots of hill work!! Just a few suggestions there's many other ways to build up your horses strength :))