How it works
DisplayPort links a graphics source to a screen and can also carry sound. The available picture modes depend on the entire connection: graphics output, monitor input, cable and any dock or adapter. A monitor advertised with a high refresh rate does not necessarily offer that rate through every input. USB-C can carry DisplayPort through a supported alternate mode, but a USB-C connector alone does not establish video capability.
For example, a laptop may operate one monitor correctly when connected directly but offer a lower resolution through a dock. That difference can come from the dock's display limits or shared connection bandwidth rather than a defective screen. Resolution, refresh rate, colour depth and multiple displays all affect the required capacity; changing one setting can explain why another mode becomes available.
Practical checks
Check the manuals for both ports and identify the cable and adapter models. Select the intended input on the monitor, then inspect the operating system's display settings. If the picture cuts out, compare a direct connection with the dock connection and change one component at a time. Keep a working display available before altering settings that could leave the screen blank.