It may be a watering issue, or what they're planted in. I recommend you buy some potting soil, remove the seedlings from what they're growing in now, carefully with something thin (I use a child's paintbrush, the opposite end to the bristles) so as not to damage the fragile roots, then replant into the potting soil in different pots. Make sure the pots they're currently in are watered thoroughly prior to removing the seedlings,then water in thoroughly in their new potting soil, allowing the pots to drain down freely. Don't add compost to the potting soil - unless it's been produced using a hot, aerobic method, it is not suitable for use in pots. As they grow on, you can move them elsewhere (in the ground or other larger pots) later on, but keep them watered as necessary - when the surface of the soil is slightly dry to the touch, give them a good soak and let the excess drain way from the bottom of the pots,and empty out any outer tray or pot so they are not left standing in water.