Problems with zone-transfers

[Fredrik Petersson]

Hi All!

I have two nameservers master M and slave S.

M has a public ip and are not behind a firewall.
S has a local ip and are behind a firewall with #53 open for udp and tcp.

The problem is that S doesn't update the zonefile when I restart it thus the
zonefile has a new Serial on the M.

But if I delete the zonefile on S and restart M, the S get the new data.

If I run (on S):
dig axfr domain.se @master.name.server
I get the correct data.

What should I check??
What can be the problem?

Best regards
/Fredde