This applies to Particle V1 (no USB) only.

Earlier versions of the Particle switched power on/off using the input jack. It requires a mono (TS) 1/4” plug to make the connection and turn the pedal on.  Verify that you are not using a TRS (stereo) cable.

If you are using a synth with an 1/8" stereo output, the Hosa CMP103 is a good choice.  You can also use a ⅛” TRS to ¼” TS Y cable, and leave one of the TS plugs unconnected.