It might be, that you need a certain amount of power in a nearby altar.
It is possible that NEI fails to mention it, so check the in-game witchery booklets on this.
Also, just to be sure:
1) The ingredient need to be dropped in the right order (you probably know this but I mention it anyway, for other people with similar problems reading this)
2) Make sure the dropping of items isn't messed up by magnets, hopperhocks, ender-hoppers, and similar things.