Email not downloading

Occassionally, I see an email on my phone (Yahoo Mail app) and my PC (when logged into YAHOO), but it will not download to emclient (set up as POP3). Most emails make it, but a few do not. They are not in SPAM either… just normal inbox emails. Why is that? 

Not sure, as I don’t use POP3, but maybe when you view it on-line and it is marked as read, POP3 ignores it? I suggest always using IMAP.