Pansy problem

I have pansies (and other plants) in pots on my north-facing patio and put down slug pellets on planting about a month ago.  My pansy flowers are all being eaten, leaving jagged edges once the culprit reaches the stalk.  There are no slug trails and no dead slugs.  None of the other plants or the pansy leaves have been touched, and there is no soil on the ground, so I don't think it's bird damage.  Under some of the saucers I generally find woodlice, centipedes and slugs. 

Can anyone suggest what the problem might be, and a possible solution?