Was going to ask the question, but found the answer anyways, so in case someone is puzzled by the same problem :
Currently running a human bowman with high enough related skills, i was hunting a dwarf captain of a bandit camp while my allies were busy killing and dying, i managed to find the target and started to fill him with arrows.
He ran away, i went sneaking around to find him and he was prone near a tree.
I started to shoot but impossible to hit at all, didn't made sense as i had no problem with target that were far away and i was only at 6 or 7 tiles , and the bow/archer skills were high.
I wasted at least 30 arrows, even moving to 5 tiles (not more to not get out of my sneaking status), but nothing to do, he was impossible to hit.
Then i found this :
http://www.bay12games.com/dwarves/mantisbt/view.php?id=5247And followed the comments instruction, moved a bit away and got out of my sneaking status, the prone captain then noticed me, i shot .. hit , he tried to rush to me while i was reloading, but every of my arrows then were hitting.
So if you play a throw/bow/crossbow, keep in mind that it is a nasty bug : if the enemy is prone and you are sneaking undetected, no arrows/bolts/whatever you throw will hit them, you will need to get out of stealth, so be sure to have cleared the place and got some distance, as a ranged character that get out of stealth can die very easily.