I think we are using it the wrong way which makes it too powerful. Can you give a Dragon who is attacking +2 then if he wins the challenge unkneel him then attack again with him using a different challenge then if he wins that challenge unkneel him again?

The text is: " Challenges: Kneel Astapor to choose 1 participating Dragon character you control. Until the end of the phase, that character gets +2 STR. If you win the challenge, stand that character."

So, since you have to choose a "participating" dragon, there must be a challenge in progress when you trigger Astapor. That dragon gets +2 STR until the end of the phase. It also says that if you win "the" challenge, you get to stand the Dragon. Well, "the" challenge doesn't mean "a challenge" or "any challenge;" it means a single, particular challenge - the one that is in progress when you trigger Astapor.

Any other challenge you win will not be " the challenge," so you would not be allowed to stand the Dragon after you win any other challenge during that phase.