Mom isolating the strongest kitten?

Is this common when there is one obviously larger and more advanced kitten? This nearly 3 week old is already running and pouncing, sitting, cleaning itself, climbing out of its nest box and exploring and beating the crap out of the other ones. The others are fairly normal, one is a bit behind and I'm keeping a close eye on that one actually.

Missy is continually changing nest boxes and leaving the other one behind and it cries and cries until I get fed up and bring it to her and the others... its driving me a bit mental.

Nay, it is usually the other way around, she isolates one who appears sick.  (not always, sometimes she does take extra care of the sick one).

I presume she sees him as the whom appears sick, him being the different one.

Good you are helping them, both the a little weaker one, and the extra strong one.

Are there other stress moments the mom i s changing the nest the whole time?   You do change the bedding every day, if she stays two days running in the same nest?

Good luck!

I think she keeps changing box as she doesn't feel very secure and I also think she is leaving the weaker kitten as she 'feels' he is too weak.

Does she have a room of her own complete with nest, food, litter tray?  She might need more seclusion and privacy than she has right now.  If you can work out how to get her more settled in one place she might pay more attention to the weaker kitten.

She also might come back into call which often leads to the cat ignoring all the kittens.  Don't let her out until she has been spayed, and talk to your vet about how soon he will do it.  These days they seem to like to wait until 10 weeks, but 20 years ago my cat was spayed once they were eating solids which was about 6 weeks, and it didn't cause a problem.