Okay...
Well, a couple things.
Depending on how much Coco you have on the top, that might be the issue. Coco is a soilless medium
that doesn't have any nutrients in it other than what you put in with your feeds, whereas the supersoil
has (or should have) LOTS of nutrients in it. I would think that the tap root SHOULD be down into the
supersoil layer by now but it may have had a long go of getting there. Plus, Coco is very, very good at
moisture retention, so there's no telling how much of the water you're putting into the pot is even getting
to the supersoil. Unless you're watering the pot all the way to run-off (getting excess water coming out
the bottom) at which point we can assume the supersoil is getting soaked.
One of the reasons your seedlings may be tipping over, is because the taproot is not developing in the
"blank canvas" of the Coco....
Another issue here is that Coco (as a soilless medium) desires a lower pH range than soil does...
If memory serves, plants in Coco desire 5.5 to 6.1 pH whereas plants in soil need to be in the
6.2 to 6.7 range. So I'm wondering if you're getting some stunting from that. I have experienced
severely stunted growth rates when not at the proper pH level in 50/50 Coco/Happy Frog.
My guess is that you are experiencing something similar.
Just to confirm, you are growing an auto flowering strain, correct?
Do you mind telling us which strain it is?
A supersoil mixture that is following a TGA/Subcool recipe is going to be intended for photosensitive
plants...and is going to be much stronger than what commonly is considered appropriate for
autoflowers. The "rule of thumb" for auto flowers is to start them at 1/4 strength of what would
otherwise be recommended for non-autoflowering (photosensitive) plants and work your way up
to half-strength concentrations while keeping an eye out for nute-burn on the tips of the leaves.
So if it's a recipe intended to emulate TGA/Subcool's, it's potentially 4 times too strong...which
can also stunt growth/development severely.