Yes follow them round till they produce a sample, collect up a few fresh, even sized samples from lambs in the group, put in bag/tub/glove, then store in fridge till you can get it to the vet. Most vets do same day results and will advise what to treat with if treatment is necessary.
I would do one this week- tell the vet you’ve dosed and what wormer you used. If it’s worms then you may have resistance, if it’s cocci then wormer is as much use as a chocolate teapot.