Ianbeth, since the pots are small, you will probably need to give frequent, light feeding to make sure they don't run out of nutrients, or use a timed release product. Starvation is a common cause of stunting in young seedlings. Check the temperature, too: if the greenhouse is unheated, it may be too cold for vigorous growth, especially with the Rudbeckia. You will also need to spend a few weeks hardening them off, before setting outside. I would probably start the hardening off process when the seedlings have about six true leaves, maybe eight to be sure.

I'm growing Penstemons on my kitchen window sill & they are very slow growing too! They do now have six leaves but are way too small to contemplate hardening off. I grow rudbeckias every year, but for the first time am growing perennials rather than annuals. I have put the biggest ones in the greenhouse but the rest are still indoors. It's just so b...dy cold! Be warned too, slugs LOVE rudbeckias, so you need to make sure they are as big & strong as possible before puuting them out. I've never needed to feed flower seedlings before but am about to start to try to help them along a bit.

I'll admit, I'm more used to starting Penstemons and Rudbeckias outdoors in situ, and usually the seedlings are about 7 cm across by their sixth or eighth true leaf. Maybe stronger light would help, though if that were the problem, they would also be stretching for the light.