If you're using an adapter on the iPad, maybe the adapter should be plugged in (I don't know if this will give enough power to the A port), or the adapter doesn't support enough power over the type-A port at all.
My theory is you could try out a direct type-C to type-B cable (straight from type-C on the iPad to the port on the drive, no adapter in between). It could provide enough power then.
The drive might have USB 3.1 micro-B and a cable like this would go with it to type-C devices:
https://www.amazon.com/AmazonBasics-Type-C-Micro-B-Gen2-Cable/dp/B01GGKYIHS/
Or if it's full-size B then:
The Cable Matters USB 3.1 Type-C to Type-B is specifically designed to connect current generation external hard drives, USB switches, docking stations, and port replicators to the new release computers such as the 2015 MacBook and Google Chromebook. Preserve your investment ...
www.amazon.com