Doesn't look like a lack of light to me, The fan leaves on the bottom of the plant look yellowed and burned, next set cupped but coming out of it, Your side branching seems to be coming out of it as well . New growth vigorouse, and leaves are reactive to the light, looking like you might be nearlly recovered. Key now, is don't re-burn it just give it what it needs. This doesn't mean feed it every other watering, just feed it what it NEEDS. Your soil probably has enough nutes now to carry it through veg. What has happened is early in growth you overdosed it, or caused it some stress. It shut down for awhile to recover from the boo-boo's cused by feeding a small plant or seedling, when planted in composted or formulated soil. Now just let it veg under 24 hour light. when ready to bloom ,bloom it. Also sometimes when you have a pheno this will hap0pen ,1 plant will be diffrent in some way, smaller ,, slower to veg, diffrent leaf structure ect.