Just like people, plants have different growth rates. They are living organic beings, not robots, so, to have some variation in growth rates is perfectly normal and completely fine. After a few weeks there will be no differences noticeable. Please do not place anything over your seedlings, like plastic cups, cannabis is not a rain forest plant. Placing anything over young seedlings can lead to suffocation, heat stress and elongated/stretched growth or all three of these things together, resulting in weak, spindly seedlings or even death. When growing plants, try thinking "what would happen in nature?" as this is what indoor growing is trying to provide, a piece of nature in a tent/grow room. There are certainly no little cups or mini sweat lodges that come to "protect" seedlings in nature. Also, try watering with a decent amount of plain, clean water at this early age (think "rain"). Watering with a syringe would barely be providing anywhere near enough water to fully moisten the container that the seedling is in. One good drink every few days is much better than giving a small amount of water (syringe full) often. You need to encourage the roots to grow, explore and expand. Giving only a small amount of water right next to the seedling will stifle growth, as the roots won't want to leave the small moist area, which in turn will inhibit/slow down growth. Hope this helps.... Organoman.