Why is my phone doing this?

For about the past month my phone has not been charging properly. When it’s on the charger plugged into the wall it charges very slow, but if i’m on it the battery says it’s charging, but it de charges. So yesterday I had my phone completely fixed. I got a new battery in it, got the screen fixed, and my framed fixed. They told me that Iphone 6’s are bad about using “off market” chargers so I got an apple one. When I plugged my phone into my boyfriends truck it charged perfectly even when I was on it. It charged perfect and fast. When I got home I plugged it into the wall and then it just started charging slow again, and when I’m on it, it de charges. What’s the deal?