I find the same thing (and I am referring to Wemo switches which are compatible). Echo still seems able to control them on command even if the echo app insists they are offline so I don't worry about it. On the other hand if you mean they don't appear at all in the echo connected home devices list, first try a power down & up (of both the wemos & alexa), but if that doesn't work best thing to try is a full reset of the wemos in question via the wemo app. I've heard people having issues with them, and others recommending a full wipe and reset when you first buy them as you they seem to be more stable generally then. I adopt this policy whenever I add a new wemo. I set it up, then do a full wipe, and set it up again. I've not had any of the issues you see reported on many forums, other than the odd wemo global platform short outage or planned maintenance which affect everybody. I have about twelve wemo switches.