Please try this Trouble Shooting Steps for R100

The troubleshooting for these issues as below ( can not pair/ connect, the left and right can not pair together, only one earbud paired, one side no sound) for R100 (A3981); 
1- Turn off and turn on your Bluetooth device (such as a computer, phone, or tablet), clear ALL pairing records in your Bluetooth pairing history, and turn off the Bluetooth of the connected device. 
2- Fully charge your earbuds and reset the earbuds, as per the steps below: 
* OPEN the charging case (The earbuds should be inside the case); 
* Press and hold the button behind the charging case for 10 seconds. The LED indicators of the charging case will flash red 3 times and then put away your hand on the button , wait for 4S the right LED rapidly flash white 
3- Turn on the Bluetooth of your device again and search for "Soundcore Note I or Soundcore R100" and pair.  
4- Make sure your Bluetooth device is within 1m (3ft) with your earbuds.