Silver Mound Artemisia (Wormwood) Doing Poorly

A few years ago we planted two Silver Mound artemisias. The first and second year they were absolutely beautiful and grew enormously. After that they began to start balling up in the middle of the plants. I thought it might be a moisture issue and only watered around the plant instead of directly on it. But the plants continued to go down hill. I've lost one, and the other is so small and doesn't bush like it used to. I have them in full afternoon sun. Please tell me anytning you can that may be causing this problem, please.

I am not quite sure what you mean by "balling up". I am guessing that the plants are dying in the centre. Try to dig the plants up in early spring, divide the plant into smaller pieces and discard the centre. Plant them into fresh soil or freshly augmented soil. I think this rejuvenation of the plant may do the trick.
The plant also flowers by producing tiny yellow balls of flowers that later turn to seeds. If that is what is happening and you don't like the flowers or seeds, the previously described division will put off flowering for another year.