If it's bad give long acting Terramycin injection every 3 days until you see an improvement (up to about 3 injections), and dip the foot in Golden Hoof - it's great. But if you feel you are getting on top of the problem then just give it time. Check the foot once a week, but don't keep trimming back and fiddling with it - use Golden hoof again if you need to. What you might find is that muck has been shoved right up between the hoof and the foot and until that is removed the foot cannot heal, except with the use of an antibiotic injection.
Small farmer is correct that some sheep do have a greater tendency to have foot problems than others and that it can be hereditary. However, here we allow the sheep a dose of footrot if she gets it then we see how she responds to treatment. Usually it heals up after one treatment, but sometimes it comes back again and again - those ewes we wouldn't breed from - or a tup either. As we also use fleece, we wouldn't necessarily cull the ewe, but we do have one Shetland with a tendency to bad feet who we have kept for her fleece but we keep her away from the main flock, with just a couple of chums, so she doesn't pass the rot onto the rest of the flock. Most of the time she is fine, but her feet need more care than the others.