Eating our Skimmea

Morning all from gusty Norfolk,,OK im putting 2 pics up to show the 2year old Skimmea, because of little growth and leaves being eaten i changed the soil JIN no3 and have fed both Male/female plants,one is ok one is really poor ,last night in the dark i checked twice for snails and slugs and found only one snail on the soil now dispatched,i sprayed 2 weeks ago with wash up liq and garlic which i use to good effect sometimes ,but this time to no avail, the plant is being eaten by i dont know what ,theirs no snail tracks or mess of any kind on the plants, im stumped, perhaps a complete inspection of roots and all or burn, any ideas anyone

many thanks Alan