Do you actively do anything to ensure your email app gets the latest mail, or do you just wait for it to arrive? If you just wait, look for a 'get new mail' option among the menus.

My Android email app (K-9) used to get mail by itself but after an update to Android 7 it now goes to sleep and I sometimes have to select 'get new mail' manually.

Facebook messages use a separate method to email.