There is a max of how many skeletons will spawn within a certain range of a skeleton spawner. What is the number of blocks that make up this range? Currently each new skeleton that falls into my kill zone dies due to exceeding the allowed number of skeletons within a certain area.
I'm not sure if the mechanics have changed however in older versions I used to be able to move the skeleton outside of the skeleton spawner 'area' which allowed more skeletons to spawn while still being in range of the spawner to spawn more skeletons.
Basically, I could place my guy between the spawner and my kill zone... move the skeletons far enough away from the spawner in order for even more skeletons.
Currently if I keep my guy sitting at the kill zone there's only about 25 or so skeletons to kill. I recall in older versions so many more skeletons and therefore the ability to increase your XP was much easier and faster.