Some or All Attachments Missing from Encrypted Email

What would cause some or all attachments to be missing from a GPG signed and encrypted email? In the unsigned and unencrypted email with the same attachments, all attachments are visible.

Maybe a problem in the Mail client?

There is a standard Format for Signing and Encrypting attachments (called PGP/MIME).
Some Mail clients only support “Text” encrypt / sign. This means that they just encrypt the text of the mail and replace it with the encrypted “PGP Message”.

Which E-Mail client are you using? And can you give us more details when and how this happens?