First time growing and It’s all organic!

I’ve seen this so much on the site... but I don’t see anything for 2019.
The stems are turning purple on 2 of my plants. Purps1.. which I’m not to worried about and a Nicole Kush. I have 2 Nicole Kush plants with VERY different characteristics but one of them has purplish/pink, yellowing/dying bottom leaves and some dropping.. I’ve tried everything. Bat guano, dolomite lime, plain water.. the leaves seemed to have stopped dying.. but the plant looks light to me and the stems are still purple... last pic is of the healthy Nicole Kush.. no purple stems pr yellowing.. please help!!!

Why do you have all those dead pot leaves in the top of your soil

They are the leaves that I have pruned or has has yellowed n died off...I just let them fall on top of the soil. Why is that relevant? Can you help?