If you haven't received a reservation notification in the app, first make sure you're not inside the app: in that case you won't see the notification, but you'll find the new reservation on the Homepage, in the "Last events" section:
If you are not inside the app, you will see a notification on your screen, as in the following example:
Also, make sure that your Internet connection is working properly.
At this point, it's crucial to check how you've set up notifications for the ZaK app on your device. So go to the Settings section of your device, search for the ZaK PMS By WuBook app and, inside, click on the Notifications item.
Check that notifications and sounds are enabled. In the left image you can find an iOS example, and in the right image an example of Android:
Another cause, also, could be the outdated app. So go to the App Store/Play Store from your device and search for the ZaK PMS by WuBook app. Make sure it is updated: if you see the UPDATE button, it means it is not. Click the button and download the latest version.
You can also try uninstalling and reinstalling the app if the problem persists.
If even with the above suggestions you can't solve the problem, we suggest downloading the app to another device and logging in with your credentials. Then close the app and, from your computer, try to enter a manual reservation.
If you receive notifications correctly on the latter device, you can assume that the problem is related to your device and not to the ZaK settings. So make sure that the notifications are correctly set on your device and that the memory is not full.