If the pots aren't too large, try standing them in a tray of water which should stop snails and slugs crossing on to them. I suppose you have checked under the rims of the pots where very small snails and slugs hide during the day, both inside and outside the pot. Some pots have rims that curl around on the outside and they get in there.
Otherwise, it really isn't completely mad to get up at three a.m. once or twice and see what is busy munching them, and pick them off. A couple of good forays and you'll get rid of them.