Trouble charging with iPhone 6S smart battery case

Hi, I own an iPhone 6S and with it I bought the Apple Smart Battery Charging case, about a year and half ago.

I've noticed that when charging the phone with the case attached, sometimes they will not charge together (when the phone is in the case, it'll charge for half-hour to few hours and then stop). I've noticed that recently the problems seems to be getting worse and worse. Before this problem would occur maybe once every two to three weeks, now it seems to be occurring two to three times a night.

What I've noticed is that when the two are charged separately there is no issues, the issue only occurs when both are together and charged at the same time. At first I thought maybe my charging cable was going bad, however I've tried new cables (third-party and new apple cables) and the problem is still there. I thought maybe the battery in the case or phone was going bad, however the phone will still hold a charge for a day and the case with phone is the same(which makes me think both are still in almost new condition).

I've gone to google with this problem and found I am not alone with this issue. There are many post of this issue on forums and reviews, but I've yet to find a solution to the problem. If anyone has a solution please share.