Thanks for bringing the issue to us via this post and email. Kindly be noted that we've well received your email and replied to you for more details to assist you further. Could you please kindly check and solve the issue with us together?
Meanwhile, @ndalby & @nigelhealy, thanks for your professional suggestions.
You are right, the PowerCore 15600 requires a 5V/2A wall charger for recharging. And normal recharging time would be 8-9 hours. If the wall charger offers 5V/1A, it might be insufficient to recharge thus the battery might not be fully recharged or it takes a very long time.
Wish all of you a great day!