This tends to be a problem with a recommender's email account set up.  The emailed invitation is either blocked or automatically filtered into a junk mail folder.  An applicant can check the status of their recommendations, resend a recommendation request and even update a recommender if the recommendation is not yet started via an applicant's Personal Activity page.  This can be accessed under the Apply Online section of your online application homepage and by clicking on the Review Your Submitted Application/Manage Your Recommendations link.