I adjust my trigger level at the bottom of my signal+ but it doesn't trigger. Why not?

I adjust my trigger level at the bottom of my signal+ but it doesn't trigger. Why not?

The trigger level is the horizontal line indicator used to visually represent where the trigger voltage level is. But there is also an invisible line, called the guard band. The guard band exists below the level indicator for the rising edge trigger, and above the level indicator for the falling edge trigger. 

Hysteresis guard band

The best way to explain the guard band is to imagine a dashed line about two minor divisions or so, below the solid trigger level line you see when adjusting the Level knob. See Figure 1 for an illustration. The signal must first pass through this dashed line to arm the trigger and finally pass through the solid line to generate the trigger event. This prevents noise from generating a false trigger. The hysteresis guard band is why you can trigger very near the top of the signal (rising edge) and cannot trigger near the very bottom of the signal because the bottom of the band has gone beyond the signal. Just the opposite is true for a falling edge setting. 

Figure 1: Illustration of Trigger hysteresis

Noisy signals

Noise Rejection coupling has a wider guard band that assists in triggering noisier signals. Conversely, if you really need to trigger at the bottom of a signal with positive slope you might try using HF Reject coupling. This filters out high frequency noise and allows the scope to tighten the trigger hysteresis.