For many of our journals, the editorial staff has often forgotten that after selecting a reviewer, they need to click on the e-mail icon to actually send the e-mail. You may want to check with those performing this action to verify that they are completing this step.
If the action of sending the e-mail has been completed, does it show up in the article record's history as a sent e-mail? If it is showing up as a sent e-mail but is not being received by the recipients, there is also a possibility that there is a problem with their e-mail client. Knowing more about what's happening will be very helpful for resolving your issue.
Electronic Publications Manager
Office of Scholarly Communication and Publishing
University Library System, University of Pittsburghvgabler@pitt.edu