Getting the best accuracy from a crossbow

What are the main factors of getting a crossbow to be more accurate. I shoot a non feathered bolt rather than the traditional fletched bolt would this play a part in accuracy, i also read somewhere that improper cocking practices and the prod being a fraction off center can be another factor and the placement of the string contacting the rear of the bolt, too high up or down to it rubbing along the top of the tiller bolt grove or trigger release. Any other suggestions hints and tips to improve accuracy.

Yup, all those matter, but don't forget a simple practice of holding the crossbow with limbs level.  If you bank your limbs right or left, even a few degrees, that will tend to throw your projectiles in the direction you're banking (like an airplane turning) banking (Kentucky windage) can be used to offset a crosswind, rather than holding your aimpoint off into the wind.
Also, make sure of your bolts.  They must be absolutely straight, including the head, and your feathers should all have the same pitch and be truly opposite one-another.  No dihedral or anhedral.
Outside banking the bow to one side or the other, the most common error I have seen in new shooters is pulling the bow slightly off center, so one limb is stressed a bit more than the other... or simply having the bow off-center in its mounting.  Try lining up a shot at your target, then lift your head a bit so you can watch your bolt's flight. If it yaws, porpoises, or whirligigs, you're doing something wrong. Check prod centering, check differential drag of string on one side or the other, check to see you're really pulling evenly.  Put a bit of colorful thread around center-string to tell you if you're getting it square.  That oughta be enough to start.  Geezer.

Additionally: having a prod that's loose in its mounting can result in whirligigging, and bolts without nocks (none of my medieval bolts use nocks) should be perfectly square, no rounded edges or slightly angled butt, whether right/left or up-down.  Bolts with eroded butts are more likely to misfire or fly erratically. I have seen un-nocked bolts with a very slight hollow cut across the butt to help center the string and prevent its sliding off and mis-firing.  Generally speaking, if your string is pushing the bolt's butt at or near its center, and the butt has the right profile, you won't have misfire problems.  And of course a rigid string-snubber over the lock will go a long way toward solving misfire problems, even if the lock is a piece of crap.  Carry on!  Geezer.

Thanks for the tips and info. It seems that a crossbow needs more skill and attention than using a firearm  rifle.

Also keep aiming after you released the lock. don't lift your head straightaway.
This helps to keep the crossbow steady until the bolt completely left the crossbow.