Diagnosis
The Bluetooth chip in the toothbrush handle has a weak signal and is easily confused, especially if you've tried pairing it before. It needs a full reset of its connection memory.
How to Fix It
1) On your phone, go into Bluetooth settings and 'Forget' or 'Unpair' any device named 'Sonic-Fusion' or 'Waterpik'. 2) Close the Waterpik app completely. 3) Now, reset the brush: With the brush OFF, hold down the 'Mode' button and the 'Power' button together for 10 seconds until all the lights flash once. This clears its pairing memory. 4) Open the Waterpik app fresh. 5) Put the brush handle right next to your phone (within a foot). Follow the app's pairing instructions. It's crucial the brush is near the phone during this first pairing.
If That Didn't Work
Make sure the brush battery isn't critically low (blinking red). A low battery cripples Bluetooth. Charge it for an hour and try again. Also, try deleting and reinstalling the Waterpik app—sometimes old app data causes the hang-up.
Heads Up
The Bluetooth is really only for syncing data at the end of a brushing session. Don't expect it to stay connected in real-time like headphones. Pair it once, use it, and it'll sync when you open the app later near the brush.