Yeah as the horns grow you do sometimes get bits of blood or raw skin at the base, and the flies are attracted to it, and the sheep gets struck. Also they will use their foot to scratch off flies, thus making the skin raw.
Crovect only works on the area that it applied, so you wont have applied it to the head I suspect. Also it doesn't always work above 25 degrees C.
Remove all the maggots and eggs and clean it up, then cover in Stockholm tar.
If i see any blood on any of my lambs horns I get hem in and bung a blob of stockholm tar on it, it seals the wound and the smell keeps the flies off for a few days.