Hello j_cocker,
Let's start by disabling the "Only when there are changes" option. Also, unmark/mark "Enable notifications" option.
Try the following as well.
1 - Please check app settings -> Mail, receiving.
Some phones (my Huawei Honor) may "remove" the app from "Doze Mode exception" after a few days.
If the app needs to be excluded again, there will be a green panel /prompt on that screen. In addition, "scheduled mail checks" should be enabled.
2 - There are phone manufacturers that pre-install their own "battery optimizer" on their devices. Some examples: Xiaomi, Huawei, ASUS. Perhaps your phone has one of those "battery optimizers" built in too, and if it does (best check with the manufacturer's support), please white-list the app there too.
3 - Swiping the app away from "recents" ("running") apps can, on some phones, also prevent scheduled mail checks.
4 - Your phone's "background data" should be enabled (if not, the app won't be able to access the network, which it needs to check mail).
5 - A "third party" battery optimizer app such as Clean Master also can prevent Aqua Mail from checking mail (making it impossible for the app to work in the background).