Paul,
This isn't the answer to your question directly, but if time and money allow I would remodel the part in Pro/E (Creo Elements Pro) so you have modifiable geometry when other things need changing or fixed instead of whittling on a chunk of dumb geometry. It is usually quicker to remodel a part when the design intent is clear or established and thus you have a better idea of how you want to control the part and features. You also have all the main geometry in the STEP file to use as your guide.
One possible direct fix is simply creating the same feature on the other end of the part if the mirror feature is not working for accuracy issues or other reasons.
Good Luck,
Mark A. Peterson
Design Engineer
Varel International