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With that much nutritive organic amendments, you'd have to be pretty artful with your watering to avoid sour soil below your roots. Seedlings often have strong taproots, which could possibly explain why they're happier. Where the roots are in a container can completely dictate whether a plant is growing like crazy on the available nutes, or bumming out. Same mix, just different size plant to pot and location of roots and watering style.
I'm not disagreeing with PSJ, it could be fungnats. It's just that I have them all the time, and never, ever saw any signs that they were damaging my plants. But, there could be a perfectly logical reason(s) for that. For example, maybe my watering style never allows them to build to the level where they do damage, wheras yours does. Fungnats biggest nuisance to me is when they fly up my nose, I hate that!