Slugs don't nip off the top of the stem. They eat the tender leaves. If you have slugs you will see slime trails. Something else is snipping off the tops of your plants. Could be any number of critters. Do you have or are you willing to invest in an inexpensive game camera? That would quickly identify the culprit.
Elizabeth - or Your Majesty
Living and growing in Lafayette, La.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant. ~Author Unknown