Q.how deep to plant peonies
Peony will not bloom. I have four plants and only one blooms and it only had two blooms this year. Others have none.

The eye buds of the Peony should be above the soil level. Peonies that are planted to deep will likely not bloom or show few flowers.
There are several other requirements for Peonies to bloom, the links below will help you pinpoint the issue.
Remember it can take at least 3 years for newly planted Peonies to bloom.
https://www.gardeningknowhow.com/ornamental/flowers/peony/peony-care.htm
https://www.gardeningknowhow.com/ornamental/flowers/peony/why-your-peony-buds-but-never-flowers.htm