Comes back to my original point – how good are they at doing a worm egg count and what have they taken into account when interpreting the results?
A faecal egg count is very easy to do so most labs offer the service, but it is very easy to do it badly. Believe me, I could go on at length on the sources of error in a worm egg count, but I’ll spare you.
If your lambs were wormed two weeks or less before the samples were taken, the count shouldn’t be low it should be virtually zero (technically if you have less than a 95% reduction in flock average egg count you have anthelmintic resistance). If the average egg count isn’t very low (perhaps 50-100), then I would change wormer families and retest some.
Fluke may not be relevant for your farm. If it is, did the lab do a separate fluke egg count (rather than just look for fluke eggs during the worm egg count)?
However, is it possible that your lambs are suffering from the after effects of an earlier infection – a bacterial/viral infection or coccidia? It is not unknown for the gut lining to become damaged to the point where the animals never really recover their full productive potential.