Not detecting some instances of viruses

[Jon Bates]

I'm having trouble whereby only SOME instances of the same virus are being
identified by ClamAV.

The virus is exactly the same type every time, but only some get detected -
the rest are sent on to the user!

There is no pattern that I can see - Zip files (containing infected exe),
and plain exe files have been allowed through.

I've subsequently scanned the users mailbox on the server using clamscan,
and it DOES detect the email! For some reason, when it is scanned when the
message is received, it's not detected. 

Any help would be appreciated!

- Jon Bates