a couple questions

It's been a couple weeks and my plants are doing ok but some have alot of growth and a few dont. It's been 3 weeks in veg so far and I just changed my nutrient solution (using the GH) to 5-0-10 ml per gallon M-G-B. I tried using 5-10-5 ml and the plants didnt like it at all. I'm also using a Mg supplument MagCal 7.5ml per gallon on my ebb flow setup. I think I had too much nitrogen as I can some nute burn (I think) on some leaves. But what do you do about the leaves covering the other plants and how long until I should switch to flower. Last question I have right now is why doesnt my biggest plant have any lower growth?

i can help with two of them, well alot of plants that are tall dont grow well on the lower parts of the plants b/c of lack of light down there, as for flowering that is 100% up to you. Depends on how tall u want them to be and space for them, and if they are healthy.