top heavy seedlings

Throughout this growing season my seedlings - both flowers and veg seem to get top heavy and flop over. I am following the planting depths for everything but it still happens. Its not light levels because it still has been happening recently and has happened both in the house and gh
Is it because -
I left them too long before potting up or/and
The initial seed sowing trays were too shallow- used mostly the standard trays/multicells

Thanks
dmg

Sorry aunt sally,
I just potted up the last of the seedlings like that - lemon sorrel
It as if the compost is too heavy for the roots- its has just been b&q multi purpose for everything as there is a small amount of stem/root? above the compost and then several of the leaves start

dmg