You might want to try distilled water, some seedlings can be very sensitive to the traces of chlorine in water from your faucet.
Too much sunlight can also sometimes burn leaves of sensitive small plant seedlings because of the ultraviolet, or the excessive heat (which can cause the leaves to dry out). You could try an LED grow lamp.
Make sure the temperature is not dropping below about 62 F at night.
Instead of a small bag, I would create a larger space inside for the seedling by putting two clear plastic disposable cups on top of each other. That should still hold the humidity.