As long as there are reservations, DME picks up at each resort every 30 minutes on a schedule. For example, resort X has scheduled DME pick-ups at :05 and :35 after every hour, resort Y has them at :25 and :55 after every hour, etc.
But like I said, this is only if there are reservations. So let's say no one checking out of resort Y that day has a flight leaving between about 3:10 and 3:40 p.m. ... that means there will be no DME bus swinging by to pick people up at 12:25 p.m.
And yes, you can pre-arrange with DME for an earlier pick-up. And as you already learned, if you don't pre-arrange it you may or may not have any luck, either because the bus is slated to be full OR there is no bus picking-up at that time.