There might be too much fertilizer accumulated from each watering. You could try washing a few plants as a test. Just water well for the water to run from the bottom, and set them somewhere where it's breezy and warm to not sit wet for too long.
You see gnats flying around in case they are present before doing anything about it. If all fails just take the plant, wash the soil and put it in new potting mix, preferably something different to be sure. In case it is an early case of damping off however, even moving won't probably help.
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