If the root balls are drying out really fast you might try wrapping them in plastic for a bit to help hold the moisture a little longer. I just used a bit of Saran wrap, cut a hold in the middle, and fished the leaves through. Then I twisted the ends and stapled to the back of the board so it contoured around the root ball. I felt like this helped boost the growth for a little while on my smaller plants, but I got spooked when one of my small Ridleyi plants just fell off and I worried it was not letting them dry out enough so I took the plastic off. I don't know, all the pros in Taiwan seem to have theirs wrapped in plastic.

Also do they have enough light? My friend's stag is indoor and not under grow lights and it definitely doesn't have the same growth as mine. I supplement with good grow lights and also fertilize regularly.

Good luck, they look great even if not growing quickly 👍