It does seem that the bluegill aren't as easily caught as they used to be. Same goes for the redear.
I feel like it's just that the lake is continuously changing, and gets a little more clear each year. The old places where I used to catch are empty, but I have adjusted my tactics and now catch more deeper than before.
Plus, many of the areas where I used to catch fish shallow have gradually silted in, and the available buck brush gets to be less and less each year. As shallow cover becomes less abundant, I think the fish adjust by staying out in deeper water instead.
I think the way to stay on the fish is to continually adjust to an ever-changing lake. The fish are still in there, you just have to adapt your tactics.
If you're fishing the same spot/dock year after year, then you probably are going to see your results continue to decline over time.