Depending on the condition of the plant when you bought it and planted it (B&B, container, bare root), I'd say that is the reason for the poor performance. You probably have circling roots which are ill-inclined to grow outward to exploit a greater soil volume, while slowly strangling the remainder of the plant below ground.

On an entirely different note: you have shown us pictures - but those are NOT Viburnum dentatum. Those are one of several tri-lobed Viburnum species which are commonly available:

• Viburnum trilobum
• Viburnum opulus
• Viburnum sargentii

You can try to fertilize the poor performer. You can go heroic and dig it up, and try to disassemble the poor root system. It will be simpler to select an entirely new plant, and put that one out of its misery before it becomes a vector for further problems.

Thanks for the info! Much appreciated.