4 servers OES2 on Netware (6.5 sp 8)
Novell DNS and DHCP
2 (pairs of) DHCP (clustered) servers. One DHCP service seems to update DNS correctly, the other is intermittent.
Each of the 4 runs DNS, not clustered - one Designated Primary
DHCPSRVR.NLM Version 3.14 30 October 2008
NAMED.NLM Version 6.07.09 29 June 2010

DNS Zone set to accept updates from any of the addresses used by servers running DHCP - each possible host, the DHCP service addresses and the Cluster master service addresses

DHCP ranges configured as either Dynamic DHCP or Dynamic BOOTP and DHCP, all set to the same DNS zone, all set to always update DNS.

Examples from DHCP range x.y.142.50-x.y.142.255 (subnet x.y.142.0/24)
For a selection of DHCP leases 'Last Used:' today some have updated DNS and some haven't
x.y.142.120 Last Used 11:00 DNS correct
x.y.142.122 Last Used 06:58 No DNS entry
x.y.142.125 Last Used 07:33 No DNS entry
x.y.142.126 Last Used 03:07 DNS correct
x.y.142.128 Last Used 08:30 DNS correct

In DHCPSRVR.LOG there are intermittent lines:
ProcessDNSUpdate: zone here_nhs_uk.DHCP doesn't have DNS server address
which seem to indicate the lease granted immediately above has not been registered in DNS.

Where do I look to find the cause of the intermittent dynamic update of DNS and the fix?