Why do some of my peonies flower and others don't?

https://www.gardeningknowhow.com/ornamental/flowers/peony/why-your-peony-buds-but-never-flowers.htm

You can buy a product called Super Thrive. It is a plant hormone.

they need a lot of sun.

I have several very OLD peony plants in our yard which were here when we moved onto the farm. My grandmother also used to raise them as did my aunt in Michigan. Now being in SD, and away from my Gram's green thumb all of these years, I was faced to figure out what to do with the same issues you are. The local nurseries all said that the plants needed the ants to crawl all over the flower bulbs to produce good flowers. Every season though all I got was shriveled up little brown nubs of dried up balls and green leaves with loads of ants. I finally decided to attack the ants at the base of the plants first thing in the spring with a PVC pipe laying on the ground with a liquid ant trap inside of it. I used TERRO, the same that I use inside my home in the cupboards. Sliding a trap into a 4" wide piece of pipe keeps the trap dry. I was amazed at how fast the mound began to die off! I had to change the trap 3 times that season, but the flower blooms on each plant were huge! I had more than enough to do arrangements for the house AND to give away! No more ants for me!

Thanks I will try, but right next to those that are not flowering are those that do flower. These plants are only about 3 years old.

There are some right next to the ones that are not blooming that are blooming, but I will move that one to get full sun for the whole day to see what happens. Thanks.