Our vet told us that although lungworm will be cleared by a wormer, they can come back within 3 weeks. So from your dates, it's possible that it is lungworm, the Rycoben worked, but they've reinfested. In general sheep don't show much clinical sign of lungworm, so if they are affected it might suggest there is something else going on too, which is making them less able to withstand the lungworm. Is it possible they got a bit of pneumonia in winter and it's left them with susceptible lungs?