We bought another pair of Ryeland ewe lambs

about a fortnight ago, and one of them had the runs after we collected her. We were advised to worm her, and we did so, and the rest of the flock with Panacur 2.5%. We've clipped away all the messed wool around her bum, but its still not very pleasant. The rest of her new flock seem ok.
The grass in this field is significantly longer and richer than the grass the new lambs were used to.
One of the Rams in a second field away from the first now also has the runs. He has had no contact with the first, and has also been wormed.
Has anyone got any ideas or suggestions?
Thanks!