Need a Post-Mortem on disappointing grow - Maybe transplant problem?

I just made my first (really disappointing) harvest that seems to have gone wrong in the transplant stage. The plants - CH9 Jack and CH9 Cluster looked healthy but very stunted - they only grew to about a foot (I did some LST, not sure how tall they would have been if I left them alone.) I thought maybe there would be a lot of stretch during flower, but all of a sudden the hairs started turning orange and it was clear that they weren't getting any bigger.

I only got a few g's dry from each plant. When I pulled up the plants, I realized that they never filled the pot - it was basically the same root ball from the solo cup. It didn't look like the roots even tried to grow - the plant just popped right out of the pot.

I transplanted when there were lots of nice white roots sticking out the side of the root ball, and made sure that there was good contact from the root ball to the new medium - 50/50 Pro-Mix and Perlite with a few handfuls of compost. This is all under 8000 lumens of 5000k for veg and 2700k for flower.

I was on a tight budget and didn't have the set-up I wanted, but I can't think of anything that would cause the roots to not grow at all. This is my first herb grow but I've done thousands of transplants as a farmer. Any ideas?

Thanks for your answers, and sorry for the late reply.

I'm pretty good about keeping the soil dry, and usually only water every 3-4 days.
Temps are 70-75.