Seedlings with poor roots

I sowed a few brassicas (kale and cabbage) at the end of summer - quite a few weeks ago….and I put them into my brassica cage last week.
When I transplanted them I found that there were virtually no roots in the pots (they were all the same). The leaves were quite healthy but under the compost they were very poor. I’ve left them in the floor but I can’t see them doing anything at this late stage.
I’d started them in modules mid/ late August, then potted them on after a few weeks.
Any thoughts on what hinders root development? They were in my potting shed so temperatures have been reasonable.

Totally agree with mum. We had loads of stuff sown into small cell trays that suffered the same and once they were put into different trays with quality compost they recovered almost immediately.
A lot of the compost available is worse than useless for sowing or transplanting.

Yes I would think so as in my experience pests are sometimes less of a problem with later sowing.
Over watering can be a problem though and damage any development of the young roots.
If there's no sign of insect/pest damage and the seedlings have not been over watered I would think that the compost could be the problem.