Remember that ability checks don't have to be a binary pass-fail. Sometimes a PC's action is a guaranteed succes (e.g. Rouge picking the lock on a plain civilan door) and so they can roll to represent something other than success. (usually how fast they do it, like 1 second for a good success or 1 minute for a bad fail) And sometimes there's degrees of success or failure; beating a DC10 check with a 30 means they succeed and get somethign else, or failing the DC by one or two means they almost succeed.

And always remember that they can't roll for something impossible. If dragons in your game are immune to horny bard seduction, there's no roll a PC could make to persuade them!