Notifications arrive through a background connection between the phone and a push service. Anything that suspends that connection to save power will delay alerts, sometimes by hours.
Aggressive battery optimisation is the usual cause. The phone decides an app is rarely used, restricts its background activity, and messages then appear only when you open the app manually.
Focus and do-not-disturb modes are the second cause. A schedule left enabled, or an app not on the allowed list, silences alerts without any obvious indicator.
Finally, permissions matter per app and per channel. Many apps split notifications into categories, and one of those can be switched off while the rest work.