I have used the root makers before… but don’t have much success in getting a good root mass. I use the adjustable lights, heating pad etc. what’s the soil you use? I have used primarily Scotts potting soil.
Big box store potting soils don't work well with rootmakers. The idea is that roots are air pruned which forces up stream branching. You need space for the roots. You need a chunky professional type mix designed for trees. Most have quite a bit of pine bark. You can also make your own. As I recall, Whitcomb had a recipe for it in one of the papers. I just use Promix and it works fine.
When you water chestnuts in rootmakers, you totally drench them. Water should be running out of the bottom holes of the 18s. They don't like wet feet, so, you don't water them again until they dry out. When you first plant them water will run out the bottom holes quickly because the Promix is chunky and does not compact. As they grow, roots will begin to fill all those air pockets in the container and they will become harder to water. Eventually you will need to pour a little water on tome and wait for it to soak in, then pour more in and wait. You will repeat this until water comes out the holes which means they are drenched. At about 16 weeks they will become so hard to water that you know the roots have filled the 18 cell and it is time to transplant to 1 gal RB2s. You want to avoid any mix that retains water. Chestnuts want to be drenched and then dry out.
For frequency of watering, I like to "water by weight". This is describe in some of my chestnut threads. You can't water on a time table as they will use different amounts of water at different stages of growth. As you gain experience, you can tell by looking at the seedlings. If you see the leaves begin to visibly droop you've waited too long and they need watered immediately. They leaves should stand back up by the next day after drenching.
It takes about 12-16 weeks for them to fill the cell. 12 is on the early side for most and 16 is on the late side. The other consideration is fertilizer. Whitcomb recommends Osmocote and MicroMax. That works great but I found it is more convenient to just buy Osmocote Plus. It has most of the micros that MicroMax has. You sprinkle the mix on top of the cells. It is a slow release fertilizer that will release the nutrients about the time the nut had been exhausted.
If poor root development is coming from a lack of light, you may not be hanging your lights close enough. With LEDs I try to keep them about 3 " above the trees organized by height and hung at and angle. If you put express trays 2 across and 3 deep, 4 shop lights hung close together should be plenty of light for them.
Timing is a consideration. If you are growing lots of trees, it is hard to move them in and out doors to acclimate them. I start with my last frost date and work backwards. I want to transplant from 18s to 1 gal RB2 containers just as I'm putting them outdoors. For the same number of trees, the 1 gals will take up a lot more space. I work back from the date I want to transplant and begin acclimating them 16 weeks. I then back off 60-90 days to add moisture to the nuts to start cold stratification. I have an upper and lower deck. The lower deck gets a little morning sun and is largely shaded in the afternoon. I put them there for several weeks. I slowly move them to the upper deck a few at a time after a few weeks. If I see any sign of sun scald, I wait longer before moving the next few up.
Hope this helps,
Jack