There are a few reasons that the One might not be charging, and almost all of them are an easy fix. First, make sure you use the white Thinklabs charger and charging cable. Though other chargers may fit the One, that doesn't mean they charge the same. Second, the jack on the Thinklabs One is deeply recessed, so make sure you insert the plug all the way into the jack. When the One is charging, the Battery LED, shown in the image to the left will flash until reaching full capacity at which point it remains solid.
If you have followed these recommendations without success, try connecting the charger to a different power source to determine if that's the issue. If the issue persists, then the charger or stethoscope battery needs to be replaced. For more information on free repair, click here.