If you have enabled Chat desktop notifications and allow the notifications in your browser already, and yet, notifications sometimes are not showing up. You may check the following,
[Chrome browser] Check if System notifications are enabled in your browser flags <chrome://flags/#enable-system-notifications> and disable it. Chat desktop notifications work when this flag is at disabled or default.
Check notifications setting in your OS,
[Windows] Check at Start → Settings → System → Notification & actions and select the toggle next to the current using browser
[MacOS] Check at System Preferences → Notifications and allow the notifications for currently using browser