Sprout Stopped Growing - New Grower

This is my first time growing so I need a little help. I have three sprouts and two of them are about 3.5 inches and they are growing fine and the third one was growing good and it reached about 1.5 inches and it just stopped. It is in an 18 inch pot and the others are in plastic cups but i didn't think this would be an problem because i thought i would have to transplant it then but maybe this is a problem. the sprout has a light dark green leaves and all of the sprouts are in scotts potting soil and the temp in the room is about 78F and the soil is moist and not over watered. the light is probably about 18inches from it, i moved it back farther to see if the plant would stretch and it didn't seem to do anything. maybe is it just a slow grower? I'm new to this so any help is appreciated!

What have you done so far? they shouldnt just stop on you, there must have been a hicup somewhere. over water and PH issues are the most common issues.