The plant stems were eaten by cutworms. If you haven't disturbed the soil around the latest dead plants, dig around the little stumps, you will find the culpit: curled up worms that are about an inch long.

Next time, you either put down bigger plants, like 12 inches tall ones, or put a small stick about 4 inches long right next to the stem, leaving no gap b/w the stick and stem. Make sure the stick is 2 inches above and under ground. Cutworms like to wrap their bodies around the stem and eat it thru, thus the name cutworm.

Interestingly enough, I think used coffee ground kills cutworms. I had a slug problem this spring, so I put down a generous amount of UCG around some gourd seedlings to try to save them after sustaining quite a bit of damage. Also, I covered them up with cut 2-liter soda bottles. The next morning, I found a dead cutworm as I was removing a soda bottle. Unless I am proven wrong, I like to think that the UCG killed the cutworm as it tried to dig under to get to the seedling.