Usually, when your phone is not using fast charging even if your charging brick can supply it, it's not a matter of a wrong cable. First, you should check whether your phone supports the same fast charging standards as your charging brick - for example USB PowerDelivery or QuickCharge. If they don't, that's the cause - your charging brick will then default to slow charge, which is around 5-10W. If, however, your charging brick and phone do support the same standards, the fault might lie in the software. It's either caused by your phone trying to protect its battery and slowing down your charging (it usually happens when your battery is above a certain charge %), or because of overheating. You might also check the port on your devices - if there is some debris there, it might make contact between the pins difficult and result in slower charging speeds.