Q.plants are turning brown and not flowering
Many of my plants are turning brown and not flowering or flower for a few days then turn brown. Those that flower turn brown very quickly. There are also marks on the leaves. Is there a treatment for this?

This could be signs of fungus disease. Can be common this time of year.
Damp conditions or little air ventilation can be contributing factors.
Trim away any diseased material and dispose of.
You can treat with Neem Oil, this can be beneficial as a treatment as it works as both an insecticide and fungicide.
https://www.gardeningknowhow.com/plant-problems/disease/plant-leaf-spots.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm