The answer is something simple but isn't easy to correct - it is lack of light (assuming we are talking about growing on a windowsill indoors here? If so read on, if not go to para 2
). Some seedlings seem more prone to going leggy than others and they reach towards the light and even if you keep rotating them it just isn't enough. They are then weakened and never do as well as sturdy plants grown outside or in a greenhouse where they have full light from all directions. Courgettes and squashes are notoriously bad for going leggy and so are tomatoes.
But I'm a bit confused as you may have meant outdoors in which case they would have been too hot in those mini greenhouses and they think they are in the tropics instead of Tassie....
Maybe they still aren't getting enough light - I would take the lid off the punnet pronto and see what happens, and if they are still too leggy just sow some more direct in the soil, or in the punnets without the lids on.