Or: "How do I get more out of my crossbows than three volleys?"

I'm playing M:TW (without expansion) and I thought it would be a good idea to bring some crossbowmen (CBM from now on, in my post) along in the early stages of my campaign to deal with Royal Knights and other armored bullies, but I usually can't get them to fire off more than 10-20% of their ammo, in battles that last a decent while even.

I usually place my CBM in front of my spearman so they have a clear shot, but the enemy usually charges after they receive two or three volleys of crossbow-bolts. Since CBM aren't that fast, I withdraw them quite quickly, but then they are positioned behind my melee-units, and stop firing.

If I withdraw the CBM to my flanks, with the idea to line them up again for supporting fire after the melee clashes with the enemy, they are usually attacked by enemy melee (militia or so), so I have to withdraw them. Sending cavalry after the charging militia results in a lot of dead horses, so that's not a good option either.

Any way, at the end of the battle my CBM have hardly fired a shot, which isn't exactly what I have in mind when I bring them to the battlefield. So, what is the most efficient way to use them in battle? Where do you place them, in regard to the rest of my formation/army? Is it worth it to bring them in the first place, or should I rely on militia to deal with heavy cavalry?

Any advice would be highly appreciated!

Cheers,
Vlad