This is an established plant, was here when we bought this house 5 years ago. It hasn't bloomed. Last year we were suggested to put rusty nails under it, which I did. This year it has loads of blooms but they only opened around the outer edges. Why didn't the whole bloom flower? It is planted near a Linden tree, which is on the east of it, and there is a storage building to the west of it, so it doesn't get too much sun, yet enough to make it wilt if it isn't given lots of water. I live in southwest Missouri. I have never removed the old stems that are left after frost.
Why Do the Blooms on My Hydrangea Only Open a Few Flowers Around the Edges
Certified GKH Gardening Expert
You may be lacking phosphorus. If there is a lack of phosphorus in the soil, plants will have little to no blooms, or poor blooming. You can either use a phosphorus rich fertilizer or add some bone meal to the soil. These articles may help as well: https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/hydrangea-not-blooming.htm
As for the pruning, this article will help for future reference: https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/prune-hydrangea-bushes-hydrangea-pruning-instructions.htm