POP not downloading all emails!

POP not downloading all emails!

My POP3 download has stopped downloading random emails, but they are arriving through IMAP connection.  This appears to have started (stopped) yesterday afternoon.  What's going on that certain emails aren't being downloaded?  I even tried forwarding the missing ones to myself, but they still won't go through.

And, with that said, is there a way to "flag" an email message on the ZOHO server so POP3 will re-download as if new, hopefully without messing with the received timestamp?

Before anyone bounces on that, I don't want IMAP on that device because I want a "permanent" copy of the emails. 

Help!  I need to receive all emails on all devices !

Harry