DP is right.....transplant to fresh soil or flush your soil to restore proper ph. Your runoff is low......and is locking out your nutes. This may well be due rootbound issues....your plants look very happy and healthy in the pics and may well simply need bigger pots with fresh soil.

I personally would tell you to go buy 5 gallons pots at wallyworld or something. If you flush and restore proper ph runoff and your plants are still losing leaves.....I'd vote for rootbound.

Fix your ph though.