I would imagine yes - it's possible everything is sold out. Not only is it Tinkerbell weekend which is one of the larger race weekends at DLR and is very heavily attended, it is also Mother's Day weekend, so unless they just haven't released the dates yet (which could be a possibility), I would imagine onsite is completely sold out already.
Update: I threw in the dates for the weekend after just to see if they had released the dates yet and they were showing up as rooms available so it is looking like the weekend you were wanting has all been sold out.