Allot of sweeps won't mirror if they include a guide curve. It's really tricky working w/ them sometimes. My current part includes 10 vents, and then I have a mirrored part that also includes 10 of them. Each vent requires two sweeps to create them correctly, and allot of playing to get those to work right. The first half of the vent created fine, patterned, and mirrored okay. However w/ the second sweep I had to create all 20 of them individually.
Your best bet is probably to just play around w/ it some and hope for the best, or send it off to your VAR for some help. There are times when it just ISN'T going to work, and other times it's just tricky. Good luck, but whichever way it turns out you might be able to use that experience to come up w/ a new way of creating it so that it will mirror next time.