Why won't my Scentsy warmer melt my tart?

I recently purchased a Scentsy warmer and have just tried it out. I'm using a 1oz wax tart in it (not Scentsy) but it is NOT melting the whole thing. I've had it going for 3 hours and only half has melted. Is there something wrong with my bulb maybe? Or do they just not get hot enough to melt very much wax at one time? If that's the case, then why do they have such a HUGE bowl? I don't get it...