Look for small green caterpillars - if you find them then spray with soapy water.

They are often easier to find with a flashlight at night--caterpillars are masters at hiding during the day, but at night they come out to eat.

possibly slugs/snails too.

Must be the dreaded slugs/snails, the one reason I cannot grow them.! Have tried and given up.

Put down rolled up newspapers or small boards in the afternoon. The next morning, look under them, and scoop up and bin all the slugs and snails that took refuge there before dawn. After three or four nights of this, you will have decimated the population, and greatly reduced the damaged.