you may need to also check the "F: ammunition" assignment in the military screen. You can see at the left a list of all the squads you have plus an extra "squad" at the top named "hunters" at the right there's the type of ammo assigned and a number "bolts (100)" that means that all the hunters will divide 100 of any type of bolts among all of them, if you also have marks-dwarves squads with the same setting, you can end up being swimming in bolts and your dwarves actually equipping just a few or nothing at all.
For instance if you tweak the settings a little bit and let's say you change the "hunters" to have "bone bolts (200)" and you have 4 hunters in your fortress it would mean each of those hunters can equip up to 50 bolts. If you have other squads competing for the same bolts I don't really recall what happens, I guess that if you actually have 400 bone bolts in your stockpiles and additionally to the 4 "hunters" you also have another squad with 8 marks-dwarves, I think the 4 hunters get up to 50 bolts each reserved for them and the 8 marks-dwarves get up to 25 bolts (notice how the individual allotment changes depending on the number of dwarves in the squad) reserved for each one, but if you had instead of 400 bolts only 250 in your stockpiles, maybe the first "hunter" squad gets first pick for the allotment, getting their 200 bolts from the stocks and for the next squad only 50 are left so they'll have to do with that, either by equipping the first 2 dwarves with their full allotment of 25 bolts each or by dividing the 50 bolts between the 8 marks-dwarves, or maybe the 250 bolts get divided between all the competing squads?
An usual problem is people making a marks-squad with 10 dwarves and leaving the default setting of 100 bolts for the squad, then complaining because each dwarf is only picking 10 bolts.