What's going wrong in my greenhouse ?

Hello and welcome to the forum

Your squash look the worst, I wouldn't worry if it's only the lower leaves, but if it's the whole plant then it may be sun scorch. Have you got any shading on your greenhouse?

The tomatoes look like they have magnesium deficiency.

This often happens just when you start feeding tomatoes especially if they are in growbags or containers. It is easily treated by spraying with Epsom Salts (ask for Magnesium Sulphate at the chemists - this is the cheapest way to buy it). Dissolve ½ oz in a pint of water (20g/litre) and apply as a foliar spray each day for a week. They will recover in a few days.

The chilli/pepper plants have Red Spider mite. It is a real problem, one I have at the moment on my peppers and chillis and very difficult to control. Last year I used a spray but I'm not keen on this on edible plants so I'm trying to keep them humid by spraying with water - it's not working very well up till now. There are some organic controls (predators) but thats expensive.