There could be reasons if your DeWALT battery won't charge.

The first reason is because the DeWALT battery is not fully slot into the charger.

Make sure to make the battery fully inserted to the charger slot.

The second way it to reset the battery.

This can be done by jump start the battery.

To do this, you need another DeWALT battery.

Connect the B+ terminal of the DeWALT battery to another B+ terminal, and the B- to another B- of the battery.

When connected, press the battery indicator to make sure it is receiving power.

Just jump start it for few seconds and the reset is done.

Now you be able to charge your battery again.