That wattage might be low for an iPad Pro 9.7" - [link|https://support.apple.com/en-us/120548|see here for details on the supplied chargers for apple devices] - which indicates the supplied adapter is rated for 5watts.
2x possibilities I can think of:
# The charger is not putting out enough current to charge the battery - use the original iPad charger (or any USB charger that rated for at least 5watts)
##2. The battery is dead and not accepting a charge; in which case a replacement is likely best option. Can probably find a repair shop if you're not confident to do it yourself - or if you're up for it there's a [guide|118536|repair guide here]