DNS stopped propagating

[Andrew Buenaventura]

We are currently hosting some sites in our servers.  We told the owners of
the domains we are hosting to point the primary DNS to our IP and the
secondary DNS to our ISP's IP.  We requested our ISP to host the secondary
DNS but it was only acted upon 5 days ago.  After they began hosting our
secondary DNS, one of the domains we are hosting complained that some A and
CNAME records are not being seen on the internet.  We did an NSLOOKUP and
found out that there are some DNS servers that can recognize ALL the said A
and CNAME records, some servers can only see a portion of it, while some
can't see the domain at all.  I have already appended the SERIAL and
restarted BIND and nothing has happened yet.  What do you think is wrong?
Another thing, is it okay for the primary and the secondary DNS servers to
have the same SERIAL?