Your blueberries take a dive
The Result: Sunken berries.
The Fix: Nothing brightens a bite of a summertime muffin quite like the burst of a fresh-baked blueberry, unless you discover the poor things have sunk to the bottom, where they have congregated into a mush.
The cause of sinkage is in a sense the season itself: In the heart of the summer, fat, ripe berries may be more dense than batter, causing them to drop.
A dash of flour will help blueberries defy gravity for the very simple reason that the flour makes them stick to the batter and stay put. Just toss blueberries with a tablespoon of flour before folding in. But use flour from the recipe—don't add extra; that will keep your ingredient ratios even.
As always, be gentle when mixing the muffin batter. As batters are overbeaten, they can thin out, exacerbating the problem and producing a poor crumb as well. If your batter does seem a bit thin, try sprinkling some of the berries on top just before baking.