How long ago did you plant the shrubs?

You need to amend your soil.  Google how to improve clay soil.   Organic matter works well.  If you have a small area you can also use sand - a  ratio of 1:1.  We have clay in certain areas of our yard. 25 years ago we amended the soil with compost and sand then immediately planted shrubs, which are doing well.  Anything we try to plant in front of the shrubs does not prosper ( honestly it's too much work to amend the soil again).  I suggest you amend the soil, plant flowering perennials, and plant annuals in containers.

I asked how long the shrubs have been in because I don’t see too much wrong with the pictured plants. The perennials look very healthy. Of course, adding organic matter would always be useful but I’m wondering if the OP has unrealistic expectations of the speed of shrub growth.

I have seen soil amendment recommended here for amending clay soil, which is bagged pine bark fines. THe reason I think is that pine bark has stuff in it that I don't totally understand but keeps it from breaking down.

I looked and looked but never found any until this year. This is also the first year I ever thought about gardening as anything but a June 1 event for planting annuals. Now that I went to the same garden centers in April, I see they have soil amendment. When I picked up my first trunk load, they said it sells out early.

So there you have it, it is available but only the early birds know about it.

Oh, also leaf waste as amendment is supposed to also keep the clay from reestablishing a solid clay mass, although I don't know how long it lasts compared to pine bark fines. And compost is also supposed to be good for coating the clay molecules with hummus as it breaks down, which also keeps it from adhering back into a solid clay mass.

In my rain garden I amended to clay soil with a lot of straw and compost which has make it more porous.

@floral_uk the flower beds came with the house, about 12 years ago, I’m the 2nd owner, been here for 2 years.
If I’m to amend the clay soil, do I have to take up the soil and mulch first?

No, you don't have to lift anything. If that were my bed, I'd start laying compost between the plants. The plants look healthy from here.

You don't want to interfere with the roots of your perennials; but you could fork in some amendments. Myself I'd use mushroom compost if you don't have compost you made on site. Spread the compost and punch holes with a garden fork. When it rains or you water; some of the compost will run into the holes. You can do this for bushes and trees also. Spread compost over a lawn out beyond the trees drip line. An inch of mushroom compost will disappear into grass if it's growing.

It is always more reliable to get this kind of advice locally….but in view of the time the beds have been in existence i would think starting fresh would be worth it …digging up the whole area as much as possible and removing enough soil to make room for the enhancements..even setting the existing plants aside in containers so the whole area can be re-worked…..i have found that a few pots interspersed to be used for annuals are an easy way to have lovely flowers as early as it is possible …i usually go to walmart and get the pretty hanging pots of mixed flowers and just drop the whole thing in the empty pots for the summer…instant flowers…mixed colors..for the whole summer

In the future, choose plants which are happy in your clay soil. Do a little research in advance of your trip to the nursery rather than impulse buying what's in bloom. My Virginia red clay is the stuff of which red brick colonial buildings were made and it supports a thriving garden. I certainly didn't add the volume of soil amendments recommended above. A short list of perennials that dig my red clay includes:

echinacea, rudbeckia, achillea, paeonia, hellebore, sedum, anemone, nepeta, aster, chrysanthemum, leucanthemum, solidago

As others have said, top dressing with compost or other organic matter will add humic acids to your soil which will improve soil structure a bit. The plants look happy and so I wouldn’t worry. Boxwoods tend to be slow growing, and rhododendrons and azaleas are shallow rooted, unlikely to be finding the layer of rock, etc to be an issue unless the drainage is really poor which doesn’t seem to be the case. There are azalea cultivars bred to be small, so you may have one of those.

Since this is a bed right along the foundation, some of what you encountered my be a result of the builders who often backfill around the foundation with whatever is available. You may find that farther out from the building there is less rock and perhaps less tightly compacted soil.

How much sun does this spot receive? The slow growth could be caused by a lack of enough sun light.

There seems to be enough sun to keep dianthus and lavender happy so I doubt that is the issue.