Bio pellets in a Vertex reactor

[slough shark]

Picked up the 1.5l Vertex reactor here on Canreef. Now, the reactor has a pump (obviously) and in the chamber there is the bottom plastic filter, a rod up the centre and a top plastic filter, all housed inside a stand alone clear plastic cylinder that fits inside the main chamber.
So here is my question:
I dumped one bag of bio pellets in the reactor and its tumbling but just.... the pellets are sort of clumping together at the top and not tumbling like I've seen in other similar systems..... Do I need a bigger pump?
Anyone have any expierence with these things?
Im a little iffy to dump another full bag in if I can barely make one tumble is all.
Any help would be much appreciated.
derek