Certified GKH Gardening Expert
It looks like an Annabelle that has not flowered completely, or is not flowering properly.
Annabelle's have somewhat flat blooms compared to some of the other species.
Perhaps they are receiving more sun than their nearby relatives? They need quite a bit of shade to flower at their best.
If they are in a different location as the others, then I would have your soil tested.
These articles will help: https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/growing-hydrangeas-hydrangea-care-guide.htm