Oscilloscope sweeps only once with DC sources and shows nothing on GND setting
Pull out the trigger level knob. This will put it into "auto" trigger mode, which basically means it will automatically start a new sweep a short interval after the previous one ends. This will give you a continuous trace even with a DC input signal.

On modern scopes that have microprocessors, "auto" now means something completely different, in which the micro searches for a set of vertical and horizontal settings that give a "reasonable" display (YMMV).

The "Auto Level" button does something different: It uses a pair of peak detectors to determine the swing of the signal, and places the trigger level at some intermediate (50%?) value.