NAT and changing password

We use one single domain under 10 sites.
PDC and 1 BDC on central site, 1 BDC on other sites.
Each site is connect via internet.
Part of the security is a NAT on a router.

Problem :
When a user want to change password in a site (except central site), the response is : unable to contact the PDC.
We find the origin : address translation.
But we never find a solution.
Someone know ?
Changing a password is possible without contacting directly PDC ?