Can't bring in reinforcements

Sometimes it happens that I can't bring in reinforcements when I tell a unit (mostly archers with empty quivers) to withdraw using Ctrl+W. They reach the edge of the battle map, but they don't disappear and the unit panel shows that one soldier is still left. He keeps trying to cross the unseen border but just can't do that and stays on the battlefield holding back a neccessary reinforcement unit which thus cannot come in. What's more, a next unit told to withdraw may successfully do that while a third is stuck again. It does not happen that often but still it annoys. Has anyone had something like that? Any recipe to aviod/mend it?

Try not to wihdraw your units in areas with trees as if one gets caught there the unit will stay on your panel as you describe. I played once a battle with the Horde in Georgia that have lots of reinfs going on. 3 Horde units got caught up in the woods and couldn't retreat for the entire battle, and so the attacking waves were much (-3 units) smaller in the coming.

You can attempt clicking here and there if the unit has been withrdrawn in the hope that the unit may manage to escape the obstacle.