Your observation is normal for the Normal Mode, which requires an AC signal to trigger. It's typically used with slow rep rate or single-shot signals where the Auto Mode won't properly sync the signal.
You said you were measuring a 12VDC power pack, so that's why the Normal Mode didn't work -- nothing to trigger on.
Edit: The Auto Mode works by automatically injecting a low frequency sync signal into the trigger circuit to provide a free-run trace when there is no AC signal. When the input AC signal frequency is higher than the free-run sync signal, then it syncs on the external signal. If the input AC frequency is slower than the free-run sync frequency, then it won't properly sync, so you must go to the sync Normal Mode.