Before doing anything else, check to see if you can push the charging cable any further into the jack.
The designer of the Thinklabs One made the stethoscope with an unusually deep jack so it won't fall off the cable. This also means that sometimes people don't push the cable all the way into the jack.
If your device still doesn't charge completely, Thinklabs may need to replace your battery. Please do not attempt to do this on your own. This service if free of charge under warranty. To send in your device, please follow these instructions.