Problems after our PDC is not online

We have 2003 AD in our company.Both PDC,BDC running 2003 Std Server. But when PDC fails or we have to shut it down for admin purposes, the BDC is not able to takeover the PDC roles. So users are not able to authenticate and cannot use resources from our system. Is there any way to check,where could be the problem? I run the dcdiag tools but all replications seems to be OK. Is there any recommendation from all IT Pros about this issue ? Many thanx ;))