BIND/DNS: Some subdomains resolve, some don't
 

I'm curious if this is typical behavior or not...

Whenever I create a new DNS zone file in BIND 9, some of the subdomains resolve very quickly, whereas others (particularly the root-level domain) do not.

For example, when setting up a mail server (on the same machine), "pop.example.com" and "smtp.example.com" will start resolving within a day or two, whereas "example.com" won't resolve for a week or two... even if all domains reference the same machine.

Here's an example zone file:

$TTL 86400
example.com. IN SOA localhost. root.localhost (
2 ; serial
28800 ; refresh
7200 ; retry
604800 ; expire
86400 ; ttl
)


IN NS localhost.
IN MX 2 smtp.example.com

smtp IN A 21.23.58.19
pop IN A 21.23.58.19
example.com. IN A 21.23.58.19

What could be going on here? Does this have to do with higher-level DNS processing, or is there something different I should be doing in my configuration?

My server is Redhat 9 (updated via the Fedora Legacy project) on an Athlon i686.