I can hear sound from left or right headphone but not both.
Your headphones are probably faulty, likely due to a damaged wire.
But first eliminate the simplest possibility - that you haven't plugged the headphones in all the way. Check to make sure you've connected your headphone plug all the way into One's jack. Note that the jack hole is rather narrow, about 6.5mm to be precise. So some plugs are a bit too fat to fit all the way. If that's a possibility, use the Headphone Adapter cable supplied with your One. Connect this short cable into One and connect your headphones to the cable. If you hear both L and R, your headphones weren't plugged in all the way.
If you're sure that the headphone plug is connected well and you still have only one side playing, check the headphones themselves. This is simple to diagnose - connect the headphones to a different sound source, such as a computer or mobile phone and play music. If the headphones work properly, move the wires around to check if you have an intermittent connection. Alternatively, connect a different set of headphones to your One and see if the problem persists.
If you have discovered that your headphones are working well and it's definitely your Thinklabs One, check out information about sending in your stethoscope for repair.