How can I make my blueberries sweeter?

what is your location so one can research the issue.

The three main factors governing sweetness are adequate sunlight, weather (especially moisture) and soil nutrition.

Sunlight-wise, is it possible that your bushes are getting less light now because nearby trees have grown and created more shade?

Weather-wise, berries are actually sweeter in drier years during ripening. The fruits will be smaller, but the sugars are more concentrated. In wet years, you might get bigger berries, but they're usually a little more bland.

Nutrition-wise, flavor can be affected not only by the levels of various nutrients but also by the soil pH, which affects how well the nutrients are being broken down and available to the plant. If you haven't done so lately, I'd run a soil test.

The ideal pH for blueberries is down around 4.5 to 5 – very acidic. Sulfur will get you back down there if the pH has been migrating upward.

Coffee grounds, tea bags,oak leaves ,sawdust,pine needles all add acid to the soil.

Thanks. I think I'll cut back the trees also. They have gotten pretty big. Does the age of the bush matter. I've had these about 15 years.