This could be due to that particular button not making electrical contact with the printed circuit board's contact pad. Especially a button constantly used such as the volume button.
I have fixed many remotes, cordless phones and the like with similar problems where a few button functions don't work. And the fix I use is to (like previously suggested) open and clean both the printed circuit board and the rubber keypad. (I just use methelated spirits). Then I dab the problem button, as well as any other suspicious looking ones, with what is called Conductive Paint which can be purchased online. I use a toothpick to apply the paint - it comes in a very small bottle and just a tiny amount is used per button.
The reason is that these rubber keypads are coated with a conductive layer (usually black) that rubs off with frequent use.
The solution above is thus just to re-coat the button with a conductive material.
Please note that depending on the keypad, your conductive paint may need to be replaced every now and then depending on use and paint adherence.
To lengthen the result, keep the following in mind : DO NOT APPLY UNDUE PRESSURE ON THE REMOTE'S BUTTONS. ONLY A VERY VERY SLIGHT TOUCH IS NEEDED.