Make sure both headphones are fully charged to ensure that the issue is not due to one of the earbuds lacking power.

Check that the contact points on the headphones and the charging case are clear of dirt, so the issue is not due to the product not being able to charge.

The LED on the headphones will show a blue light when correctly inserted in the charging case.

Try to reset the headphones if the product is fully charged but are still only working in Mono mode.

A reset guide for the headphones can be found here: Reset guide