The problem is the detent that is in the sleeve, a small gold colored pad that puts tension and friction on the stylus. Too much in 'n' out can wear this pad down or gunk it up with oil from your hands, dirt, etc.... causing less retention.
i know with the 6700, if you bent the stylus slightly, it would fit tighter in the sleeve. give that a try. but remember ***SLIGHTLY***
i haven't had this problem yet, cause i dont use my stylus too often. i've come accustomed to using my finger, unless a fine touch is needed.
or, you can try a Q-tip and a small amount of rubbing alcohol to clean the detent.
EDITED for sh*tty grammar.