I have been a bit hesitant on addressing this one. Put molasses around the plants?
I've been to east Texas picking blueberries off the bush and into a bucket. OK, I admit, along the way
a few dozen may have made their way into my mouth... Some were sweet, some were tart. It is my theory that this is the yin and yang of nature. It gives balance. If they were all sweet, they would be boring. If they were all tart, they would be boring. The element of surprise is something to be celebrated!
If anyone else has an idea regarding this, do tell! That being said, it likely has many conditions which affect it including temperature, humidity, and any stresses the plant has experienced.