After rereading this thread I think your problem might be soil or nutrient/pH etc..
I managed to poison my plants by over fertilizing them. They lost their leaves and grew really slowly. I finally uprooted a dorset naga that was only a stick with two small very weak looking leaves, and rinsed the roots off and repotted it in a bigger pot. The leaves fell off from the shock but then it regrew and now it has huge green leaves and is looking good.
You should look at what you are feeding them, the soil mix they are in, and the progress of any other plants you have in the same conditions. I'm just saying that if they germinated ok and then grew slow it might not be the seeds.