Regina, are you able to add a pic of your damaged Coleus? If chunks of leaf are disappearing, I strongly suspect it's some type of caterpillar if you're not seeing slime trails from slugs. Then can blend in very well and tend to hang out on the bottoms of leaves to hide. Their poo can often give-away their location. If you can find it, you can remove it.
I see that you have the frog badge on your profile. Please know that systemic pesticides are among the most dangerous, environmentally persistent ones that are generally the most dangerous to such sensitive wildlife as frogs & toads. Anywhere that sells garden chemicals should have an assortment of 'cide products. Read the labels to choose one said to be effective against the particular pest you have.
It might be easier/safer/cheaper to take some cuttings of your Coleus, rinse them well, and start over in a clean pot of fresh soil or other spot in the ground.
