1. Is it getting hot while charging? Beyond a certain temperature the charging will slow down automatically with modern models of chargers.
2. Try draining the battery to below 10%. (maybe 7-8%). Then start charging again - to check if it comes back to 15% and gets stuck there or goes beyond. This is to check the internal hysteresis loop memory imprint.
3. Since how long are you having this problem? Also which region?