From your description I have the feeling that you have a soil drainage problem. This fall I would dig it up and replant. On a berm if necessary to improve the drainage. In good soil I don't find I need to fertilize other than to maintain the vegetative compost component to about 5% of the soil. If I do need to fertilize I use golf ball size tablets for shrub fertilization made by Scots, place about a foot deep slightly beyond the root diameter, one on each side. Al

A berm and a raised bed are similar, I think, as I too have poor soil drainage, there is no alternative other than to go "up".

If you can not get the water out of the soil, you can not get the air or oxygen into the soil. The roots in the airless soil will rot, and they can not uptake nutrients OR moisture to feed the plant. The roots at the surface still can function and grow, thus the plant gets more and more on the surface. Unfortunately a plant can not thrive in these conditions and as you say, it gets smaller. In most cases it is easier to raise the soil, build a raised bed, or berm, than it is to drain the soil, which will usually involve installing drainage tiles, if that is possible in your gardens location. Al