the issue is likely due to overheating of the adapter, hence, due to the built-in safety mechanism, it stopped the charging process. (you can try charging from 80-90%, likely it will be able to charge to full)
use a 12w adapter would solve the issue. If you have a macbook or something like asus ai-charger, you can charge through those usb-ports too.
Alternatively, you may want to try turning on the air-conditioner, the colder the room the better. This may just solve the issue without the need for any extra accessories.
FYI, I have a 16GB ipad mini (non-cellular).