I suggest checking out the SCOPS website for the latest thinking on worm resistance. Basically you need to switch wormer groups (white/yellow/clear) as resistance builds up over time and any resistant worms passed onto the pasture will breed with those still on the pasture, even if they're not resistant, and produce more wormer resistant worms. Are you using the same field or turnout each year? If so it may be carrying a very high worm burden. Might be worth getting a FEC count done by your vet so you target the problem correctly.