A strange zone transfer problem

[Tun _]

Hi,

I am having a strange problem with my name server.  This name server
is primary and slave for a number of zones.  Out of the slave zones, a
few of them are getting below error messages.  Those slaves have the same 
primary DNS server. Other than below error my name server is working 
properly for its primary and other slave zones.

zone transfer timeout for "domainname.com"; pid 12345 killed
named-xfer "domainname.com" exited with signal 15.

The strange part is that the slave zones that are getting error are
still resolving on my name server. So it tells me that it is getting cache 
info from primary but the backup db files are not getting updated because 
above error.

I have two questions:
1. Since my named appear to be functioning, what could have cause
this problem.  (maybe some traffic are being bloack on the network?)

2. How can I turn-on a higher level of logging for named-xfer
process?  I am hoping that it might give me more info to trouble shoot.

Thanks,
Tun

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