Slave is not slaving all domains

[Nelissen Jordi]

hi,

I am experiencing trouble with one of our BIND master/slave combinations.
Our slave has been running perfectly for years, has been slaving hundreds of
domains without any problem. 

Now it stopped updating zones that have changed on the master server. If I
manually execute a named-xfer command for a certain zone, all goes well. By
looking at the debugging, I typically get the following pattern :

do_zone_maint for zone company.com (class IN)
zone_maint('company.com'); now 1015264158
qserial_query(company.com)
qserial_query(company.com) QUEUED
next maintenance for zone 'company.com' in 15961 sec
qserial_answer(company.com): [1.2.3.4] -> 2002030400

however no startxfer or /user/etc/named-xfer is observed and the slave
zonefile is not synced with the master.

Any suggestion to solve this issue is welcome.

thanks, jordi