It can be difficult to determine why your seedlings are struggling! They often give us visual clues to let us know what they need. While I do not advise over-fertilizing, adding an organic fertilizer can help give them a boost when needed. If adding fertilizer to a young seedling, wait until they have at least one set of true leaves. I suggest using an organic fertilizer instead of synthetic because there is a much lower chance that you will burn your plants.