Regarding applying periodic boundaries
 

hii guys,
I am simulating a 20 degree sector of a combustor in fluent.I want to apply periodic boundaries at the enclosing faces but fluent is giving following message
warning-differing no of faces on these threads
zone 2439 - matched 0 out of 37489 faces
zone 2453 - matched 0 out of 38312 faces
so please tell me how to proceed further.
Also fluent asks periodic zone and shadow zone so what is meaning of this shadow zone.
thank you in advance