Hi Graham and welcome to GoY. Do you start your seeds off in trays to begin with? We've noticed a real difference when we do this. Don't worry in any case they will catch up eventually.

No, i put an individual seed in a 6 or 9 compartment container and wait for germination, which they usually do, but stems are very thin and fragile looking. Whereas the ones i've just bought from my local garden centre, although in exactly the same container are thick stemmed, sturdy plants. Should I sprinkle seeds in tray first, then when germinated to a decent size, and then re-pot into the compartmentised trays. Graham Goodhead.

Can be a number of reasons. Seeds sown too thickly, not transplanted soon enough, uneven light and warmth...are they on a windowsill?
I sow my vegetables in 1/4 seed trays, about 15 seeds to a tray, and once germinated they go outside for a week before transplanting.

Hi Graham yes sow your seed in a shallow seed pan only prick out when they have their first true leaves and are of a size to be handled. Make sure they get lots of light or they will become drawn - look thin and weedy! Would help if you could change your profile to let us know which area of the UK you are in.

Growers start them in glass or Lexan greenhouses, sometimes with supplemental lights, so that they get lots and lots of light, including some ultraviolet. That keeps the seedlings nice and stout. Some growers also spray the seedlings with chemicals that keep the spaces between the leaves small, though (thankfully!) that is more often done with flowers than veggies.