I bought home a catnip filled toy once and my cats went absolutely crazy for it. Eventually, after they'd sucked the hell out of it, the effects wore off.
Then I found a box of loose catnip (within a plastic lining) that belonged to my previous cat. Although opened, it was within the expiry date. Did absolutely nothing for them. 
Want to get them some good catnip for xmas, are some types or brands better quality than others? Maybe the box of catnip had just been opened for too long?