No reason for it to be the rain, if one's blooming and the other isn't.

If they're on opposite sides of the garden, then they're probably not getting the same amount or quality of sunlight, and maybe not getting the same amount of water, and the soil could well be different too.

Peonies are usually pretty reliable about blooming, once they're big enough to start, they keep blooming every year unless they get shaded out, or moved.

Around here they're full sun plants, although US gardening books often describe them as part shade plants because in the eastern half of the country they do take part shade very well - not here. If you're in coastal BC you need to give them as much sun as possible.

If they're new to their locations they might just need to settle in for a couple years first.

sometimes, if a peony is planted too deeply, it won't flower well or at all....

This particular peony (the other one is blooming fine) is growing very well..... but no flowers. It was in it's current location when I moved to this house, 3 yrs ago, and I have done nothing to it. However, I did give most of that bed some fertilizer in May, so perhaps that is the reason. However, it looks very healthy and I'll just wait until next year!