Did you notice green(ish) layer on top of the herb soil ? I think it is a moss or algae thing (don't know the right term for it), it grow because your soil get continuously damp. The green thing won't harm your herb but the soil seems to be lack of air and thats that harm your herb. Try not to over water it.
When your herb seedlings are still young, try to keep them indoor as the temperature in your green house may be fluctuating too much and get too cold at night, they can't take it.

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