gnus not slurping mbox files

[Harry Putnam]

I've had these lines in gnus for years:

  (setq mail-sources
             '((file :path "/var/spool/mail/reader")
               (directory :path "/spool/in/"    :suffix ".in")))

And then I have procmail splitting incomming mail by provmail rules
into mbox files ending with the suffix `in'  (`in') like

   friend-name.in

Its an mbox file with whatever number of messages, there may be
several created or added too each time I pull mail from pop3 box.

gnus slurps the mbox messages but never deletes the empty file.
I've forgotten which part of the manual covers this.

That has worked for quite a few years.  But ever since I made a major
move to a new host and have to get mail working smoothly.  Gnus has
refused to slurp from the files procmail creates for that purpose.

I don't see any errors and nothing in \*Messages\* that seems to
indicate anything untoward is happening.... just seem gnus flat
ignores the mbox files.

How can I get to the bottom of what is causing this?