Catnip self-sows for me like crazy... and I collected some seeds last fall and started them with my other seeds under lights and they sprouted without any problem. Possibly the seed needs to be fresh, or maybe it needs light to germinate? I think I also winter sowed some, but so far the only thing I have sprouting in any of my 15 jugs (which probably represents at least 30 different things planted), is some Sweet Annie just beginning to show.
My cats are crazy for it too -- but that's why I have it!