Samsung S5: Getting some but not all Comcast emails

[AC Question]

I'm using the default/stock email app on my S5 with Comcast email. About 80-90% of my email messages are coming through on my phone, but the rest aren't showing up, and if they do, it's at least a day later (I can tell because they are showing up when I log into Comcast).

I can't quite pinpoint any commonality among the messages I don't receive. In fact, from a given sender I may either receive the email in a timely manner or it can be delayed (sometimes even a couple days).

I searched for this problem and found similar, but not exact, issues, none of which seemed like they would pertain to my situation.

Any help is appreciated, as I've been late replying to many important emails.

Thanks!