Bicycle Tutor

I have mounted several cassettes before and they always feel very snug on the hub once they are locked.
This time i have mounted (twice) a SRAM powerdome on Mavic wheel and it doesn't seem tight. The cassette can move a little bit around, even though the lockring is tight.
Any ideas whether a) I should be worried about it and b) how i can fix?
Thanks!
Manlio

Maybe the cassette is narrower than the freehub body? Shimano (and compatible) freehub bodies that are 8/9/10 speed compatible (so, basically most of them at the moment) are too wide for a 10 speed cassette, you need to install a spacer on the side next to the spokes. The current state (loose cassette) is not acceptable, since the chain might jump off and get wedged between cassette and either frame or rear wheel, locking up the wheel. This can lead to a crash.