ATX PSU as bench power supply. Voltage dropping issue
Unfortunately, there is not a solid standard for the ATX supplies as they evolved over time.

You may need to load all three buses. Try 1A on the 12 volt, 0.5A on the 3.3V, and 0.3A on the 5 volt bus. Older supplies even required that the loads on the 5 and 3 volt buses total to be approximately equal to the 12 volt bus to work properly.

You may have a supply which requires a 3.3V sense line to be connected to the 3.3V output to function. This is usually identified with a 20 pin connector. Connect pin 11 to the 3.3 volt lines if this is the case.