Recent versions of Android have a "feature", sometimes called "Smart Network Switching", that will ignore a connected WiFi network if that network cannot reach the Internet. The network shows as connected, and EngineDriver server discovery works, but clicking on the discovered server or Connect button results in an error like "Can't connect....".
Some devices show a small "x" or "?" on the wifi icon. Some devices have a setting to turn off this mode, some do not. Some provide a notification that "Wi-Fi has no internet access, Tap for options". followed by "This network has no internet access. Stay connected?" that can be used to disable this "feature" and remain connected to the wifi. Sometimes you have to "forget" the network, and reenter your credentials just to get the prompt.
One (hopefully) consistent way to test for this is to make sure that your device has mobile data disabled. You can enable "Airplane Mode", then enable ONLY WiFi, and try connecting again.
The next version of EngineDriver will show a message "Using MOBILE network, not WiFi. check WiFi settings", when this mode is encountered. (available in the [Beta] now)