Why my iPhone 5c is showing it's charging but doesn't? (not IC issue)

Hello,

I am into a real weird situation and i would really appreciate your help.

One friend of mine gave me for repair his iphone 5c cause it had faulty mic and home button. I replaced the whole flex cable (headphone jack, mic, charging port, home button metal contacts) with a new part. The fix was really smooth and didn't encounter any problem. Mic fixed, home button fixed. Next day i gave back the phone to my buddy (he was really happy with the result) but the day after that, he gave it back to me and said that the phone doesn't charge. I tested it again and the phone shows it's charging, can connect to pc and i can transfer photos without any problem. When it's connected the battery doesn't decrease. When it's not, it is decreasing like normal (no fast drain at all). I installed the old part again ( mic,home button faulty) and it is charging normal without any issue! I ordered another new replacement, installed it, exact same result.... I guess it's not the famous IC issue (microsolder) cause with the old part it's charging normal. I imagine it's incompatibility for some reason (but both new parts are original) ? For that thought i just ordered a new battery just in case but haven't recieved it yet. I 'll keep you updated when i recieve and install the new battery after 3-4 days but for the time being is there anything you could suggest me to do?

I have searched many days for a solution but it seems that i am the only one with that exact issue. I really don't have any clue what might cause this problem ( except from incompatibility). I am not a pro technician with any special equipment, just doing it for me and some close friends. Really sorry for my poor English it's not my native language.

Thanks in advance