My first guess is that competition from the maple roots is causing the slow development. Maples are notorious for sucking all the moisture and nutrients from the soil and making establishment of other plants difficult if not impossible. Especially larger growing shrubs that need greater root development and that are slower to establish than smaller perennials and groundcovers.

I wouldn't recommend any pruning but I would recommend babying them a bit with extra ferts and water to make up for what the maple is robbing.

Many hydrangeas can develop a leaf spot, Cercospora, usually encouraged by overhead watering. It could be that the maple when in leaf is dripping rainfall down onto the shrubs and not much you can do about that. In most cases, it is not a serious affliction and is just cosmetic damage and does not require treatment.

Take those size promises with a grain of salt. Size estimates vary for many reasons and waiting 5-10 years to achieve the size at maturity in the plant labels is common. Providing them with morning sunlight, adequate & evenly moist soil, 2-4" of organic mulch and adequate nutrients/acidity is the best you can do.

I usually do not buy specimens if they have leaf spot diseases. I have seen them for sale with cercospora (but not often) and with even with rust one time. For full prices, I want a perfectly healthy shrub. Cercospora is a youcannotgetridof it thing here (only visible in mid Summer thru Fall) so I installed drip irrigation and try to keep the hydrangeas' area clear of debris and dispose of dry leaves/blooms in the trash in the Fall.

I have not seen cercospora in the Spring ever but by July 4th, I can notice some leaf lesions that look darker green, just a bit darker. I also have a maple that drops some junk on the floor in Spring but I can remove with my hands from other surfaces. I have kept hydrangeas and other plants with roots in the top 4" away from my maple so there is no competition for nutrients. The closest shrubs are 10' away or so.

Oddly enough, Cercospora is very uncommon in gardens here. It is typically a nursery production issue that disappears once the plant is established in a home garden landscape. It is almost entirely aesthetic in its impact and does not adversely affect the health of the plant.

GG48 has the perfect weather for hydrangeas with cercospora. My hydrangea has had cercospora reliably every yrat, for at least 20+ years, here though. The previous owner left a hydrangea already infected (no idea how old it was then). :o( But taking those measures I mentioned earlier, minimizes the severity. Keeping things clean and uninfected did not cost any cash, just my time, and I always have to do some cleaning in Spring anyways...