I would say 7 days is the minimum time between wormings - but only if the animal hasn't responded to the initial treatment. Sometimes it will take 2 or even 3 shots to get rid of the problem, depending on the health of the animal and the effectiveness of the wormer. If you haven't moved the sheep off the infected pasture then definitely re-worm that lamb. Move it to cleaner pasture if you can.
You don't say what wormer you used? Depending on your area some won't be as affective as others (and no, you won't know unless you do a FEC a while after after worming).
Check that you know the weight of the animal and that you are giving the right dose, or even a bit extra.
And yes, defo, some animals are more immune to worms than others.
As for wet grass, I have started keeping some hay in the hay rack all year, which they mostly ignore, until we get lots of (Welsh) rain then they seem to appreciate the dry matter. I think that all my sheep's bums are cleaner because of it.