Thanks for visiting the Fretlight Support page. Whatever your support needs are we're here to help. We've put a lot of answers to your questions on the website.
UPDATE 2/1/19: Thanks to one of our great customers they discovered that if you "clear Bluetooth cache in hidden apps" on your Android Mobile device, the Guitar Tunes app for Android and the Fretlight work!
For reference, here is the fix in Android 8.1 (Oreo) -
Go to System Settings > Apps and Notifications > select "See All" > in the top right corner select "Show System" > Bluetooth > Storage > Clear Cache