What makes a portable-monitor setup fail even when both devices have USB-C?

The connector is the least useful clue on its own. Before buying, I now check three separate links in the chain:

1. Does the laptop port actually output video (DisplayPort Alt Mode, Thunderbolt, or USB4)?
2. Can that laptop drive the requested number of external displays in the intended lid position?
3. Does the monitor get enough power on the same cable, or does it need a second USB-C power input?

A fourth failure mode is the cable itself: a charge-only USB-C cable can make two compatible devices look incompatible.

For anyone who has debugged a portable monitor, which of these caused your failure — port capability, display-count limits, power, or the cable? I’m collecting the edge cases that deserve clearer guidance, especially combinations where the honest answer should be ‘unknown’ until the manufacturer documents the port.