iPhone won't charge

I have had my iPhone 6s for almost three years now and have recently had a problem with it not charging well. It charges sometimes. When I put it down to charge I have to plug it in multiple times before it actually starts working. Usually when it charges it works but sometimes when it charges the precent goes down instead of up. Also sometimes when I plug it in it has no indication of charging but the precent rises anyway. This usually doesn't work for very long though, and the precent will start to go down again. My charging cable is not apple but I tried my stepmom's (which I think is apple) and it still didn't work. I also have tried restarting it multiple times and cleaned both the charging port and my cable. Is there any way to fix this? Thank you