Biggest immediate culprits are:
-Soil: Bagged soil has been awful the last few years. I let my bagged soil sit in the bag in the sun for some months before I used it. If there are contaminates, etc. The sun and heat takes care of them.
-Sun: Too much sun will stunt the growth of your plants. You can either move them to an area that receives less sun or provide them with shade cloth. Likewise, too little sun will stunt growth. You can move them to an area that receives more sun.
-Water: Rainwater is best! Do you collect any? Or, were they overwatered/underwatered? Either way, try to have a soil moisture reader on hand.
-Seed started vs Bought starts: Seeds that you start are used to your area, even more so if you saved them from the year prior (they grow like monsters). Starts are hit or miss. They are used to one area and get moved to another so it may take a while for them to catch up.
-Pests: Do you check for garden pests? It's best to check early morning before it gets hot, or late evening once it starts cooling down. You'd be surprise what you find (if you don't already do this).
-Too much nitrogen: Are you using fertilizer? Too much nitrogen will encourage leaf growth and nothing else.
Yes, I know most people will say. "Oh, they're spraying!!! That's why xyz isn't growing!" But they are always spraying, you should still produce more than what you currently have.
