Is it possible that another noisy device like a halogen lamp or an old CRT TV was turned on 15 minutes afterwards? That could be what was blocking the SlingLinks from communicating with each other over your power line. Otherwise it is possible that you just have defective SlingLinks.
You can walk through two articles for SlingLInks (http://support.slingbox.com/get/KB-5000041.html) and (http://support.slingbox.com/get/KB-5000022.html). If you're unable to determine the issue through process of elimination, then it probably means you should call Sling Media's technical support.