Same issue. Galaxy S3 Classic watch. Mine is a Bv9900 Pro, but exact same situation. I downloaded and setup the Gear Wearable app, Samsung Accessory Service, and Gear S Plugin. Made sure Battery Optimization was turned off and every permission was enabled for all of these. Paired right away the very first time I set the phone up. After about 30 minutes, my watch disconnected and hasn't been able to since. I reconnect via the Bluetooth settings and it will say it "connects" on the phone, but the Gear S3 and Wearable app say it isn't there. I've tried removing and reinstalling everything but it just locks up and never finishes pairing again. I tried syncing the watch to my old phone and it worked right away. The bluetooth in my Bv9900 seems to work fine with other devices, no issues connecting to my car stereo. It's some sort of software clash.