Could it be that since they are patrolling they have no time to go gather the bolts. 1 technique is to a ammo stockpile with the metal bolts assigned along their patrol route at a spot where the civvies can restock it out of site on any hostiles.
I have seen times when crossbow squads have shot up all the bolts assigned in practice, then a patrol order comes up not allowing them anytime to refill their quivers. So while you attempt to create more for them to use, the ammo remains in the bins until the squads patrol order ends, then the idjits will refill.
Here is an ongoing experiment that is working well. I have 1 squad assigned both wooden and metal bolts.The metal bolts are along the patrol route. Instead of a CT, C or T designation, I leave it blank till problems arise. Main reason am in a iron poor biome. So when I want them to practice, will create some wooden bolts. When gobbos show, I add CT to the metal bolts, let them fire away. When they leave, will remove the CT designation, they will return the metal bolts to a bin, instead of wasting them at archery practice.