I am almost embarrassed to be asking this question so late in the game. This unit assumes a line-level input, so the sensitivity of the envelope follower and the range of turn-on thresholds is based around the expected range of signal amplitude. If a person feed a guitar directly into it, the unit will not likely be sensitive enough to turn on.
I would suggest a few things, here, if this is the case. First, try using it with a clean booster (e.g., LPB-1, MOsFet Booster, etc.) to confirm that the problem IS a question of mismatch between threshold and signal level, rather than a construction error. The signal level may still be somewhat lower than the design expects, but with the threshold set low, and your booster set high, you should be able to turn the gate on when you pick hard (make sure to set the attack quick and release long).
Now that I am reminded of it, I seem to recall that in the exchange of letters to the magazine in the months after the article was published, Jon suggested chages to the values of R7, R10, and R11 to arrive at a range of voltages so that comparator A (that op-amp section) could be made to trigger in response to typical guitar range. I wish I had saved those issues or at least photocopied the pages. Unfortunately, I don't remember the recommended values.
Another possibility is to add some gain to the op-amp stage marked C. If you put a 22k in the feedback loop and a 1k from the inverting pin to ground, I would imagine the level would be brought up high enough that the existing threshold range will work.
