my greenhouse grow

I've been growing these two plants in my greenhouse for about 6 weeks. I have a few problems so any advice would be appreciated. First.... the big bottem fan leaves keep going yellow and dying which means my plants are not looking at all bushy! There are also brown patches on a few leaves. I am now trying to flower them by covering them up at 10:00pm and uncovering at 10:00am. I'm not sure if this will work? Also, are the pots the right size or do i need bigger? Please help me as i'm a newbie!

They do have some sort of nutrient problem, not sure which one. Is that the normal yellowing? If so that isn't a nitrogen deficiency. It looks more like a phosphorous deficiency to me.

Covering the plants will work, but you need to make sure they are completely dark, no light leaks at all. When they start to bud covering them up could drastically increase humidity and cause mold or bud rot. I don't see any signs of sex yet, so keep an eye out for those preflowers.

You should move to bigger plants and a good soil mix to flower them. They are already pretty big and they look like they are going to stretch like crazy.

thanks for your help. I'll transplant them and post some more pics in a few days.
Just wondering what 12/12 actually means. When i cover them up, it's dusk outside..... so there's still a little bit of sunlight. does dusk count as part of the light stage? And does it matter if it's not exactly 12/12 (like 11/13)?
Thnakyou!

You need to have 12 hours of complete darkness to start flowering. Any light counts toward the light stage. Some strains need more than 12 hours of darkness to properly flower.

its so exciting....preflowers are showing now on both of them and are both females (i'm 90% sure anyway). I'm so attached to these plants I just dont won't them to die!! I tried feeding them with a high phosphorus fertilizer but this didn't seem to work as they are now even worse. I'm thinking it is a nitrogen deficiency. I'll post more pics in a few days.

you need to continue your veg food for about the first 2 weeks of flowering, if you jump right to a flowing food you will get a nitrogen def.

what kind of yield will these plants give me??

I'd get a fan on them to strengthen up the stems also

just some updated pics. they look alright now i think

put them in pitch blank for 12+ to start vegging, but you need to give them pitch black for 12hrs everyday for the first few weeks of flowering