You may not receive Notifications due to several reasons:
You are not subscribed to receive all notifications or notifications of a certain type
Go to File menu
Select 'Edit User Profile' option
Select Notifications tab
Check boxes of all notifications or those you would like to receive
The tasks are in status 'Draft'. Notifications are turned off for the status 'Draft'
Change the status
Highlight the task or tasks
Right click to show the context menu
Select 'Change status' option
Change status to any but 'Draft'
Change 'Draft' status settings
Go to Tools menu
Select 'Custom Workflow' option
Highlight workflow 'Default'
Double click on status 'Draft'
Select 'On' for Notification option
You make changes in tasks assigned to user under which you are logged in. In this case you don't receive notifications as there is no need to inform yourself about the changes you have made in your own tasks.