Alyssum seedlings healthy?

I sowed these Alyssum Snow Cloth in a tray of seed compost (single surface, not these modules) on 24th Feb. They germinated quickly, within just a few days, but then grew very slowly compared to all my other seeds. In profile, many of the seedlings are like McDonald's-shape 'M', with leaves pointing down. Pricked out in to these modules of multipurpose compost last weekend but they're generally not growing much. ~1cm tall. Am I correct to think they're not v healthy?! If so, has anyone had a similar experience with Alyssum and any ideas of possible or likely cause? I've several other plants growing from seed in the same room (S facing windowsill, radiator beneath) - verbena, F marigold, dahlia, sunflower, delphinium, cosmos etc - and they're generally doing really well.

Thanks for any thoughts.