You're probably over fertilizing and coleus don't respond well to fertilizer. When you feel the need to do it (which shouldn't be all that often especially if they're in the ground and not containers) - dilute the Miracle-gro to half strength. Even in the ground, dilute the fertilizer or, better yet, forget entirely.

I bought a few this year, planted in partial shade and watered them in. No fertilizer at all and they're doing beautifully.

My coleus do really well with Miracle Gro full strength once every two weeks. They're huge. If your leaves are falling off, then something else is wrong. Check for bugs or rot of some kind. Maybe your potting mix already had fertilizer in it and it's too much, or, it's too wet or dry.

If you cut them back, they will re sprout and grow bushier than ever.