Missing Notifications

[MagicSquares]

I have an Android LG G2 phone, running 5.0.2, and about a month ago, I noticed that notifications from certain apps (e.g. Words With Friends, UPS, DailyArt, Amazon Seller Central, Dictionary Word Of The Day) have not been displayed for a while. Other notifications, including systems ones, Facebook, and from an app called Ringo Lite, are stilll working fine.

I can't see anything on the phone settings that might be causing this, and I don't believe any apps I installed at about that time should cause it.

Any thoughts on what might have caused this, and how to get my notifications back?

Thanks!