Hi James, The 20:1 attenuator should not be inserted when using either the secondary clamp-on pick up or the COP probe. It would explain the very small signal you are seeing displayed !
Keep the 20:1 attenuator for primary ignition and injector work only. If you use the presets for these measurements you will see (20:1) on the left of the display in the units scale, indicating you need this attenuation for the scale reading to be true.
I hope this helps.