3 NXDOMAIN for *some* reverse lookups

Hi,

I got a zone file with hosts within 192.168.0.1/24
Most reverse lookups work, but some don't. I do not see any logical =
scheme.

Example: host 192.168.0.1 is resolved. .2 and .3 do so, too. .4 gives me =
an "Host 4.0.168.192.in-addr.arpa not found: 3(NXDOMAIN)". .5 and .6 =
work again and so on.
They are all in the same DNS-domain. Of course PTRs exist, A record, =
too.

I use bind 9.2.1

Thanks, Helmut