The Xbows always work fine for me, here's how I build their training room:
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w = wall
S = Space/floor
x = bolts stockpile
D = Door
b = bed
f = booze(drinks) only stockpile
F = Food only stockpile
Make sure each one of the Target Range is designated as a training room. On the alarms, make sure that the Active/Training is set to Two months of training with over half of your marksdwarves active for training(set the minimum to 4 if you have 6 guys on that squad for example). and set one month off-training, then two more active, and so on. Also, make sure you set each one of the the archery ranges to actually train the squad, optionally to sleep too. You already mention you have quivers, backpacks, bolts, and crossbows, so there's everything you shall have, but also make sure to have TONS of bolts, wooden ones are excellent for training, and the metal ones are of course for live action/combat, so when you are assigning the ammunition, set about 20 pack of wooden bolts for training, and about 20 packs of metal bolts for Combat. (You will notice that a C appears next to the selected bolt when it's for Combat, and a T when it's for training, and a CT when it's for both. To designate the materials of the bolts, use M.
If your dwarves are using the metal ones for training, then use two stockpiles, one for "Other materials"bolts, and the other for Metal bolts, when in training, Forbid the metal bolts, and your dwarves will be forced to use the wooden ones. Do the opposite when in combat.
Just in case, make sure you assigned an uniform that includes a crossbow to the squad.
Have fun buddy! Otherwise, FUN will have you!