Preface: not all dwarves can equip a halberd, as the minimum size necessary to wield one in even two hands is above the average size of a dwarf. In order to determine which dwarves are large enough to wield a halberd, I've been using splinterz' DT clone, which reads dwarf size and weapon raws directly.
I have an iron halberd bought from the human caravan that nobody wants to equip, and I can't figure out why. The halberd is accessible and non-forbidden, and the only free weapon in the fortress (the rest are picks and axes being used by the miners and woodcutters). The military dwarves are all large enough to multigrasp a halberd, and I've also tried getting every adult dwarf in the fortress to use the halberd.
So far I've tried 1. using a uniform that's just the halberd, being both specific and nonspecific about the material and also changing the exact match / over uniform settings, 2. assigning the halberd directly, 3. setting the proper military uniform to include 'any melee weapon' and exclude a shield, and 4. setting every civilian in the fortress to a uniform with just a halberd, and then deleting the uniform a dwarf at a time just in case an improperly-sized dwarf is blocking the slot or something.
So far nobody's claimed it with the green tick, let alone picked it up.