Buy / use an Anker Powerport with 2.4A output, like a Powerport 2.
Use a quality cable. There is a small chance the cable you're using is not working too well, even if its Anker, so try different quality cable.
After you eliminated slower charger, and eliminated potential dud cable, then email email@example.com with serial number and they'll sort you out with a warranty replacement assuming you got from Anker. The Anker advantage is that products tend to work right 1st time, if they don't they do good customer support and warranty. I not had an Anker Powercore fail on me yet but there's the small possibility.
I'd expect with good charger + cable and not a failed unit you'd be looking at 8 hours to recharge, I cannot find a recharge time online but I own the Powercore 10000 and from 1 light to full is about 8 hours.