Beats fit pro - only the right one works???

My guess would be audio settings on your source device, probably the stereo balance setting is all to one side. This works for AirPods, your Beats are likely to be similar.

If your left or right AirPod isn't working - Apple Support

Robyn_55_55 Said:

"Beats fit pro - only the right one works???: I purchased 2 pair of Beats fit pro and both sets only the right ear bud has sound. Why would only one beats fit pro work? I have reset them both and turned off my iphone as well."

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Troubleshooting Beats Pairing:

A. Start with a Reset:

Go Here: Reset your Beats Fit Pro - Apple

B. What to Try:

A. Verify the Connection:

First off, see if the headphones are is even seen: 

1. Hold Down: the Option key
2. Click: the Apple menu
3. Choose: System Information or System Report
4. Select: Audio and Bluetooth from the list on the left
5. Look For: this devices

Then...

B. Reset the Bluetooth Module:

See if Bluetooth is getting in the way here.  Bluetooth may be in a constant loop of not being able to connect.  See my User Tip: Use "Reset the Bluetooth module" to Fix Constant Bluetooth Disconnections - User Tip

C. Contact Beats:

Contact the manufacturer, and request assistance with finding a solution. Ask them for some tips on troubleshooting this, and ask if you can get a swapout. They make the product. So, they'd know an upfront answer.

Go Here: Product Help - BEATS SUPPORT - Beats