why are some of my plants not getting bigger?

Ok my seedlings are about 7 days old, this I'd an indoor growing keep my room temp at about 80 and moisture at 50%. I am running a 1000w mh bulb. I am not giving them bytes yet because the organic soil that I have them in already has some. I just got rid of gnats in the grow room by putting sand on top of soil, and putting sticky tape around the room. 3 of my plants are growing like crazy and the other 15 have not gotten any bigger in the last couple days? Anybody have any input on why this may be happening? Thanks!s

The first week to two weeks, your seedlings are establishing a root system and putting most of it's energy into that. Also, they should not be under a 1000 watt lamp this early in life. Not only are you wasting burn hours on an expensive bulb, you are wasting electricity. Have you considered setting up a couple of cheap T8 shop lights in a nusery area for cloning or seedlings? It sure beats wasting money on power and MH bulbs that lose more than half their lumen after only 10,000 hours.