AD upgrade from 2003 to 2008 - infrastructure master problem

i wish to add Windows 2008 Domain controller to Windows 2003 domain

so i upgrade AD2003 to 2008 version

i run adprep /forestprep at infrastructure master (server A)

but it keeps on saying "the infra master is another server B"

i check the AD users and computers at bother domain controllers

infra server is server A

the infra master is just changed from server A to server B

but i still cannot run adprep /forestprep