how do I hard reset an iPad Pro

if you mean force restart

Force restart iPad - Apple Support

if you mean restore to factory settings

Restore your iPhone, iPad, or iPod to factory settings - Apple Support

“Hard Reset” does not appear in Apple’s Lexicon for iPad or iPhone. Perhaps you refer to a forced-restart…:

Force restart iPad (models with Face ID or with Touch ID in the top button)

If your iPad doesn’t have the Home button, do the following:

1. Press and quickly release the volume button nearest to the top button.
2. Press and quickly release the volume button farthest from the top button.
3. Press and hold the top button.
4. When the Apple logo appears, release the top button.

Force restart iPad (models with the Home button)

Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.

If instead you wish to fully reset an iPad prior to disposal, or in readiness to be set-up again “as new”, you would be best advised to follow this process:

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

could be

dirt or lint in the ios devices port when that happen to me I clear it out with a match or toothpick

a defect cable, testing with another cable could show if it is so

a defect port in the ios device

and read

If your iPad won't charge - Apple Support

My ipad pro won't charge.

It seems to be taking longer and longer. Is there a ios problem?

Will a forced restart solve a " slow charging" problem?