I definitely haven't seen the problem you're having with charging. Here's what I have noticed:
1. It seems a bit slow to charge. I'm coming from a 3-year-old iPad 2, and it definitely seemed quite a bit quicker to charge than the N9.
2. Battery life is disappointing. I decided to get rid of the iPad because as it has aged, I've noticed the charge lasting less and less. That old iPad lasted longer than the Nexus 9. I'm going to run for a few days and keep my eye on the battery stats now that I have it set up the way I want it.
3. I found the cable that came with it a bit snug. I have a 5-port USB charger (we have a lot of tablets/phones here), and it works great with everything I toss at it. I got 5 decent-grade cables from Monoprice on it, and I noticed that when I nudged/bumped the micro USB when it was on the N9 the cable moved to side side just a pinch (like barely) and the N9 stopped charging. I straightened the cable and it started charging again.
Based on #3 above, I'm going to try a few other cables on the N9 to see if they are running loose. I have one at the office I know is snug in everything I put it on, so I'll have to brign that one home and test it to see. I really wish this thing had wireless charging like my N5. ;-)
~Gary