When I get bottles from friends or relatives, I am always amazed at the "science experiments" that occur in the bottoms of some bottles. I am also amazed at the huge variety of plain, simple CRAP some folks stick into empty beer bottles. Getting them cleaned out is all part of the challenge of brewing.
For really, REALLY crudded-up bottles, cut the end off the bottle brush and put the shaft end into an electric drill. Insert brush into bottle and turn on the drill. Bend the shaft at a slight angle, if necessary, to get all the crud in the bottom corners.
And, if all efforts to clean the bottle fail dismally...RECYCLE the bottle!