Usually a load of gunk on the slider. To properly clean it you have to take the DS apart - then you will see that the slider is just a plastic part - the actual switch is very small, remove the plastic - clean it and the case it slides against and rebuild - and stop operating your volume switch with sticky fingers!!
happened to me today. I didn't know what the problem was until I saw your post and you mentioned it was recessed. So I took a small pin and pushed the slider to one side, using the pin to kind of pull it upwards until I heard a small 'click'. It popped right back! (Although now it may be a little too loose, but oh well)