Trouble in "Unite" operation

I am designing a word antenna that is embroidered on a cloth. I get a trouble in simulation, which is the Unite operation. The HFSS notified that:
- Acis error: "Fail to make union..Acis Error 40002 - inconsistent containments on coincident face_face_ints" (6:22:37 CH Thg10 13, 2015)
- Body could not be created for part because of invalid parameters to Unite operation. (6:22:37 CH Thg10 13, 2015)

I haven't ever met this situation before. It make the two partner that need to be United disappeared.
Could you paraphrase for me why they disappear? Because when Unite two block, they 'll always be there and never disappear.

Thank you so much!