Why is my Galaxy S3 not charging?

[AC Question]

At first it charged ok, then it started charging slower and slower until I had to start charging it while off. It got to a point where it took about 15 hours to get fully charged so I sent it for repair and they told me it was the charging port so they changed it for a new one. It charged just fine until the whole thing started happening again. PS: I sent it for repair about a week ago.