Why Does a USB-C Charger Work With One Laptop but Not Another?

I have two laptops with USB-C ports and one charger, but only one laptop charges reliably with it. The connector fits both, so I expected them to behave the same. Is this usually a power-wattage issue, or can the cable and the laptop’s USB-C port also affect compatibility?

I’m trying to understand what to check before buying another charger, especially when the laptop still recognizes the charger but charges very slowly—or not at all.