iPhone Notification Issues

So my iPhone is having issues with notifications. A few months ago, I noticed some apps were only saying “you have a notification” instead of showing the preview as it always used to. Now, after turning off notification for certain apps, other apps have magically turned off their notifications as well. For example, my Gmail app is no longer sending me notifications even though I never turned them off. This issue is pretty frustrating because I have no idea how to fix it and there tends to be important notifications that I miss because of it. Does anyone have any advice on how I can fix this?

Thanks in advance!