Not an air lock in it, is there? If you run it with the filter cartridge disconnected does the impeller spin round nice and fast or is it struggling a bit? Drop it in to the water without the cartridge fitted and see what the flow's like then.
I know it sounds obvious, but our 3 Plus had a flow...
Unfortunately they've now got a bit more frisky so it was a bit of a challenge to get the photos, eh Sarah?!
I must admit though, they do look lovely, and we replaced the lamp in the tank with an AquaGlo which has really brought out their colours.