A spare charger in the drawer claimed 65W on the label but my laptop sat there charging at a sulky 15W. The laptop wants 20V; the charger was clearly offering it to something, just not to me. So I broke out the CC lines on a USB-C breakout board and put a logic analyser across them.
Power Delivery is just a conversation over the CC pin. The source sends a list of capabilities (the "Source Capabilities" message), each one a voltage and a current it's willing to provide. The sink picks one and asks for it. If the negotiation never gets past the default 5V contract, you stay on whatever USB-C gives you out of the box, which is not much.
What I found was the charger advertising 5V, 9V and 15V, and nothing higher. The 20V PDO simply wasn't in the list. The 65W figure on the label was, I assume, the sum of two ports running flat out, or just optimistic marketing. My laptop asks for 20V, gets told it isn't on the menu, and falls back to the highest thing it will accept. Mystery solved, and the charger went back in the drawer where it belongs.
If you ever want to do this yourself, the cheap CC-line breakouts and a Saleae clone get you most of the way. The protocol decode is the hard bit, but the shape of the negotiation is easy to spot once you know it's there. Most "why won't this charge faster" problems are exactly this: a PDO that was never offered.