Posted: 09/04/2017 at 07:59
It depends on what seeds you are growing but if they are bigger seeds I find most of mine grow well in root trainers ( I hope this isn't advertising ) you can buy them from garden centres and use them time after time.
I also find weather you are growing in root trainers or seed trays you will get better roots forming if you stand them on gravel, I think the moisture in the gravel promotes the roots to grow longer.
I also wonder if you are potting up or on too soon and the roots haven't had chance to grow.
i hope this is of some help.