Q.I Have 4 Peony Plants That Are Slow Growing And Yellow.
I have several peony plants in my yard. One section I have 5 plants and 4 of them are slow growing and fairly yellow, but one is thriving and looking great like the rest of my peonies. Could it be my soil or a bug problem? The plants have thrived for the first few years I had them planted there. They get full sun. And plenty of water. Last year was the first year I started to see an issue. The foliage stayed slightly red and didn’t fully green up like they should.

Certified GKH Gardening Expert
Have you checked for insects? I would do that first thing. It could be nematodes. Here are more issues to check:
https://www.gardeningknowhow.com/ornamental/flowers/peony/foliar-nematodes-on-peonies.htm
https://www.gardeningknowhow.com/ornamental/flowers/peony/peony-botrytis-control.htm
https://www.gardeningknowhow.com/ornamental/flowers/peony/caring-for-peonies-in-spring.htm