Bulking up seedlings

Hi,

I've been trying to grow various flowers from seed as I wanted to have a cutting patch at the allotment, but I'm struggling to get them to progress to a decent size. After nearly 2 months, some are still only an inch or so high and pretty feeble looking. Can anyone give me some tips on how to get seedlings to fill out - I can't plant them out as they are now, they'll get eaten in seconds! I've tried potting on, and pinching out (where there are enough leaves). They are all in my mini greenhouse in the back garden - I've got chrysanthemums, nigella, cornflowers, poppies, amni majus, zinnia, echinacea and larkspur.
Thanks!

It's the coldest Spring for years, which hasn't helped. If it's any consolation mine are the same 

As long as they aren't still in teeny modules (in which case they need potting on) give them a dilute feed and pray for some warmer weather 

There is no feed in seed compost, so those need to be potted on.

Lack of feed, light, cold nights, and overwatering, can all make things grow slowly.

I would take them out of the mini greenhouse, and give them dilute feed every time you water from now on, as other posters have said.

 Even some MPCs have few nutrients, and it sounds like your plants are in need of a good meal