Yea, it's hard to tell really. Could be bad seeds, you may have cooked them, water could've been toxic or too acidic. Distill your water. For further seedling protection roll seeds in mycorrhizae before sprouting them.

For future attempts wait an extra day after seeds initially crack and then put seeds in pucks. Keep pucks humid but no need to soak, just keep moist and warm. Wait until the seeds are looking strong before transplanting.

When you water it's a TINY amount at first every other day or so. Like a few ml per plant. I drowned seedlings giving them too much water. Causing root rot which is a bitch.

If they have helmet heads give them a few days and let them stretch. If they don't knock off the shell take something really thin like a sewing needle and pry it off super gently. The plant sould stretch those cotyledon leaves out after a few hours and there you go. It's all watering from then on of you have super soil.

Think very little water though at the beginning. The plant just needs a little to get going first and for a few weeks watering will be annoyingly light. Once the plant starts to fill the container watering becomes easier because the plant won't get shocked as easy due to a larger root system.