The leaves on my plants get brown spots and I cannot find any insects on them. I end up with just the veins of the leaves. To be safe, I have sprayed with a insecticide, but still have the issue.
This looks like Leafcutter damage which is a green caterpillar that feasts at night. You have a few options but since you haven't spotted the critter I would go straight for the Pyrethrin or neem oil. Both are organically derived pesticides and wont harm your plants or beneficial insects.