Can you still detect the device remotely?
Try to force close all the apps that are running in the background then re-launch the Wemo app. Circle back if setback recurs.
Wemo Technical Support
You think the background apps are interfering with the Belkin app? Why do I think that's silly? Or is your app really that fragile? I do appreciate the "circle back" irony - just like how the icon circles until it reports the device is not responding.
I haven't actually tried this myself - it's not my job to test it - it's Coleen's... maybe she'll try and comment. But it does clear up many issues in a poorly design app like this one and certainly it won't hurt anything. In fact it's SOP for the support team. Go into app settings and clear the app data, or uninstall/reinstall the app. On android the app manager is the easiest way.
Make sure you're phone is connected to the same SSID and wifi band (2.4Ghz) as the light switch, then start the app. It should find your devices again and the lightswitch will be unhidden. If it doesn't find any devices and tries to get you to set up a new one your phone is connected to the wrong part of your network.