I Clik my senior sheep as soon as the fleece regrows to around 1cm long, which is when the maggots can cling on. Lambs are Clik'd in May, as soon as I see more than 6 greenbottles in a day. At this point in the year you could probably get away with using Crovect, which isn't as long-lasting. Have you actually condition-scored your sheep by feeling their spinal processes? They may look round, which is partly dependent on their breed, but not actually be carrying that much across their spinal column.