Seriously this is so infuriating, nothing I do seems to get any consistency with using the archery range. After the incident I reported above where they actually started using it, they stopped and no one has ever gone in there again, never. I've tried everything. I've made a one person squad, that dude literally has never trained whether I set to training or inactive. Never. Archery Ranges are quite broken it seems.
Let me try to help you understand how this works.
You cannot make them practice any more than you can make them eat, or drink, or throw a party. It is something they decide to do by themselves.
The first step to understanding this is to start calling archery practice "practice" and military training "training" because they are two different things. Stop fighting this in your head. Training has nothing more to do with practice than hauling does. Scheduling your dwarves to train and wondering why they don't practice is like scheduling them to haul rubble and wondering why won't train. I know it's not intuitive, but training means the same thing that it does for your axedwarves, speardwarves, and every other military unit, dodging, blocking, etc.
The second concept: your fortress will create tasks and dwarves will accept those tasks if they have the appropriate labor enabled. These tasks have priorities, so higher priority tasks will be accepted before lower priority tasks. If a dwarf has no task available to him, he will be idle until a job comes along.
Concept 2.5: dwarves will also create their own tasks, like eat, drink, sleep. These are very high priority tasks and have no labor associated with them so you can't stop them from accepting them.
Concept 3: If a dwarf is a member of a military unit and he is idle, he may decide to practice with his weapon. The likelihood that he does this depends on one of their mental stats, will or self-discipline. If he has a melee weapon equipped he will go to the barracks and do Independent Combat Drills. If he has a crossbow equipped he will go to an archery range and practice.
So if you want to maximize the chance that your marksdwarves practice you must choose dwarves with high Will, disable all of their labors including not scheduling any training. This will make them idle most of the time. They will still eat, drink, sleep and a few other things that will take priority over practice, but eventually they will practice...
Unless....as is the case I suspect here, they run out of bolts. Bolts are secretly claimed by units that use them. If you have a marksdwarf squad of 10 and you assign them 25 bolts for practice and 25 for combat, they will instantly mark 500 bolts in the fortress for their use. Even though it looks like you have a ton lying around in your stockpiles you really don't, they are claimed. Hunters will also claim bolts. If you want a lot of practice going on you will need to create lots of bolts. Think 5000 or something like that.
I know it is confusing, there are a lot of factors. I haven't mentioned having the range configured properly because you had them training already so that's probably ok.
I hope this helps.
Edit: typos