Sounds like you might be overwatering the plants if there is a spot on the stem that looks pinched and dark, especially if it's near the ground. Weak light makes them stretch and get leggy. I use Pro Mix for starters as rich mixes tend to burn the seedlings, I also don't water from the top, instead I water into the tray under the plants and don't leave any excess in the tray after a few minutes of soaking. Too much love kills faster than not enough