A problem that you could encounter is that the involved users in an issue (for example assignee, watcher, etc.) do not receive email notifications. The possible reasons for this happening could be two.
Global email notification settings
The settings of the email notifications could be global and on a user level. Firstly, let's take a look at the global. Please go to Administration (on top) - Settings - Email notifications. You may check some parts of the available options in the screenshot below. They include core Redmine email notifications as well as RedmineUP plugins' email notifications.
User-level email notifications
When you have already set the global settings, then please also check and set the user-level email notifications. Please go to the Administration - Users - [select the user]. And here, you could find a couple of personal options for the notifications as shown below.