A lot of my friends started out really lackluster and ended up becoming really involved in debate. The reason, mostly, was they became self-motivated; they saw all their close friends were winning, didn't like that they weren't, and decided to change that. Often, this often involved their partner (who usually carried the team) dumping them and finding a better partner; this usually made them wake up to the fact that it was sink or swim time. Since they usually had the tools and now had the willpower, they swam.
I think that's one reason our club was really successful in creating motivated debaters. Our coach wasn't overbearing. We chose how much time and effort to put into debate. So when kids decided to put in effort, they did it out of their own free will, not because someone forced them to.
At the same time, though, it doesn't work for everyone. Some people just don't like debate and won't like debate no matter what. Sometimes it's an attitude thing (the parent pushes really hard, the kid hates the parent, so the kid pushes back really hard). Sometimes it's a maturity thing. Sometimes it's just an inherent dislike for the activity; not everyone is meant to be a debater, just like I'm definitely NOT meant to be a singer