Ok, so I had the same problem a few years back when I tried to start a lot of seeds. This year I thought I'd give it another go. I planted a bunch of seeds in 6-cell packs back in May.
Most things have sprouted, BUT they're "just sitting there" on their primary leaves, for nearly 4-weeks! And it's been very warm (80's & 90's daytime/50's & 60's night) and they've been getting regular water. They're in a non-temp. controlled greenhouse with it's side panels lifted/removed for air circulation.
The only things (out of 20 or so different plants) that I've been able to transplant, since they've "grown on" are Hibiscus, Datura, "Basket of Gold" Alyssum, and Hyacinth bean. A selection of things "just sitting there" with their "primary leaves" would be Basil, Balloon Flower, Henbane, Ma Huang, Passiflora, Heliotrope, etc. Is it normal for some plants to basically go dormant on their primary (not "true" leaves) for weeks at time after germination? Thanks in advance for any clues!