Casinos regulated by gaming commissions (as all US based casinos are) can not deny you legitimate winnings. The gaming commissions are there to make sure that the casinos and the public play fair, however not all winnings are legitimate, and when that happens the gaming commission will generally side with the casino.

Casinos want to pay you. They are legally required to pay you, and it's in their best interest to avoid bad publicity. But they will follow the law exactly. Most of the reasons why a casino may not be able to pay you are easily remedied.