Planting too deep can cause water to collect and sit on the crown causing crown rot, but as far as I know, that will not cause a plant to stop flowering. In my opinion, it doesn't look like yours are planted too deep. I had a similar situation recently, and it turned out to be 2 different cultivars mixed together. One bloomed the first year, but the other took more time to settle in. This year when they both bloomed, it was obvious that I had 2 different plants in the same hole.
