I had trouble keeping the sphag in the smaller pots moist so I have experimented by moving them into various size compots and bigger pots. The pots all hold more sphagnum so that helps a lot - with humidity too. Keeping humidity levels in the 50%s has been impossible these past few weeks and the seedlings have definitely suffered.
I also disposed of about 20 laggards - those with hardly any leaf or root growth. I had a remaining 25 or so in a tray and these are transplanted direct into compots. Hopefully the refreshed sphagnum is going to give them all a boost.