This tends to be a problem with the recommender’s email account spam or junk email filter settings.  The emailed invitation is usually blocked or automatically filtered into a junk mail folder.  You can check the status of your recommendations, resend a recommendation request, and even update your recommender choice if the recommendation has not yet been started via the Personal Activity page.  You can access this under the "Apply Online" section of your online application homepage by clicking on the Review Your Submitted Application/Manage Your Recommendations link.