Block for arrows?

Has anyone had this happen? You create a set of plans with many leaders in the drawing. You do a plot preview and everything looks fine. After you
plot it out you find a few of your leader arrowheads are rectangular boxes. I have tried to delete them but the blocks seam to jump to another leader. I have tried to move them out of the way and replace them but they tend to jump to another leader. I have contacted AutoDesk and they don't know what it is.
Can anyone help with this problem?
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