I have two weird Ble connection problems.
The first situation is:
If I leave the screen(like click phone back button) after I complete the connection.
When I enter the app again.
Sometimes I can't find any devices in my BLE listpicker.
The second is:
If I repeat the steps of connection and disconnection several times.
Sometimes it will be connection failed because the BLE listpicker is empty like the first situation.
Now my solution is restarting my phone(HTC U11 and SONY).