I found a bunch of old discussions (spring 2011) about markdwarves who won't reload after using their first batch of ammo. The bug is here on the bug tracker:

http://www.bay12games.com/dwarves/mantisbt/view.php?id=4084

... and here on the discussion board:

http://www.bay12forums.com/smf/index.php?topic=78556.15

I have tried all the recommendations on those pages: disbanding and reforming the squad, deleting and re-adding the ammo types in the military screen, choosing "iron" instead of "metal" for a material, making hundreds of new bolts and not putting them in bins, stationing and unstationing, etc.

My marksdwarves are all nicknamed ROTCM. Only one or two of them will pick up bolts (those ones quickly became legendary because they're the only ones doing any work). There's a caged goblin on z-4 or z-5 that I use for target practice (right lever to raise protective bridge, left to open cage) and the dwarves just stand behind the fortifications whenever I let it go.