THis is probably old hat to the experienced, but it caught us out!
We had a batch of flystrike earlier in the summer (not surprising with all that heat) and everyone was treated (fleece cut away and maggots removed) and then crovected to kill any remaining maggots. Having had problems with crovect (and ectofly which I understand is the same thing) earlier in the season (I was told by an apparently well informed observer at our local show that the chemical in crovect has been evaporating in the heat before being absorbed - hence the low effectiveness. Don't know it is true, but it explains the situation. Anyway, treated with Clik as per the instructions (shoulders, spine and around the backside) and all seemed week.
On detailed insoection this weekend when we still had a few flicking their tails we fould a number with flystrike again. However, in every case it was low down and underneath, between the vulva and the udders - exactly where the clik would not have directly hit! Another lesson learned for next year, swipe them underneath as well as on top!