Instead of being evenly distributed throughout a batter, fresh or dried fruit (berries, raisins etc.), chocolate chips and other add-ins, often sink to the bottom of the pan during baking. Some will even burn on the bottom of the pan making the recipe inedible. 

Sinking add-ins happens because the batter is not heavy enough to suspend them. Thicker batters, such as Pound Cakes, work well. Some bakers recommend lightly dusting berries or other add-ins with flour to slow them during their trip down; that works, too (sometimes) but, it's really the thickness of the batter that stops them. 

If you have a thin batter and want to prevent the add-ins from sinking, use smaller ones. Remember to gingerly fold in all add-ins AFTER the batter has been mixed. They will also suspend in the batter better.   

Berries - whether fresh or frozen, I always crush a quarter of the berries first, and then quickly stir them into the batter. (FYI - you will get color all over the batter). I then quickly fold in the remaining whole berries. The Jordan Marsh Blueberry Muffin Recipe is an example of this.

Raisins - DO NOT plump (rehydrate) raisins and all dried fruit before using in a recipe because you'll make them heavier. Or, use a lighter and smaller dried fruit such as currants, dried cranberries, etc.