Hoping the specs are close to the iPhone adapter.
In my case my watch is only charging, If the charger is not to strong. If I take my normal charger for my mobile (2A), nothing happens
If you are charging from a "wall charger" or battery pack then the charging indicator is much more subtle and the whole screen does not change like it does when connected to a computer. Assuming the contact with the charger is correct, might be worth double checking this.