How are you asking?
I don't like to nag them.
I ask them politely once. They ignore me so I say "Hey, c'mon now" and then if they ignore me again, they are going to work their little behinds off until they do what I want and only then do they get any release.
For backing, I like to use that to my advantage. There will be many people who disagree with it because some horses will resort to backing to get out of working but I have no had that issue.
So if the horse balks. I stand at their shoulder, cluck, and ask them to move. If they refuse to move I may ask them to disengage their hindquarters once or take a step back to refocus them on me. Depending on the situation I may give them a small tap on the hip with a dressage whip while still at their shoulder. If they ignore me again, I turn and face them, make myself big and scary, and then send them backwards across the arena. Fine, you want to back? We will back.
They usually look at me like I have gone completely nuts and then I can feel them wanting to give up. When they're showing signs they're ready to start paying attention to me, I will try again. They only get 3 chances, I refuse to nag.
It's the same in the saddle. If I ask them to go once with my seat and leg, and the horse refuses I may give them a tap with my heel and cluck. If they don't respond they will get a kick/over and under/pop with a crop. I don't stop kicking/tapping/using my over and under until they trot a pace. When they trot, I stop kicking/tapping/whatever.
They soon learn they're not going to get to test my limits and get away with it.