Are online HMRC Authorisation Codes being delayed?

Has anyone else noticed that the online HMRC Authorisation Codes are not arriving with clients.

 

HMRC have told us that the codes are sent out by First Class post.

One was issued on January 12 and the other on January 16. Neither have been received by the client.

 HMRC have confirmed "issued" does mean "put in the postbox" and we have checked they used the correct address in both cases.