charge phone when it is turned OFF ... does it charge?
yes = you have many background apps, wifi, roaming, location, etc on and possibly screen brightness max/adaptive. turn them off.
no = either faulty charger or faulty device. - try different charger. if a different charger works, bin old one.
do not charge your phone using a computer ... it will only slow charge and if it is on, it probably wont charge at all.