I assume by mono you mean running out of the Left/Mono output jack to my amp. Or is there an internal setting I'm not seeing? My playing technique isn't the best but it's clean enough.
No, I mean you cannot play chords. Monophonic input is required. That is typical for most gear that tracks input notes.
I use a GT-10 (the guitar version, since I play an 8-string guitar which covers the full range of electric guitar and bass, as well as an Ibanez SR506 six-string active bass and a Cort Curbow six-string bas), but wanted to comment.
gastric's post regarding getting better tracking is spot on. I own a ton of synth pedals, and there's a few more things I do to get great tracking:
I turn down the tone knob all the way. The synth pedal figures out what it should sound by what's coming into it, and if I have higher harmonics in the input, that can confuse it. Better to have no ambiguity in the pitch.
I also palm mute (more harmonic and inharmonic reduction), and make sure I only have one note ringing at a time.
I crank the bass' volume when putting it through a synth. That gives the synth maximum information.
I don't use compression before a synth pedal. Compression makes it hard for the synth to detect a new note being plucked/picked.
Your comment that you play cleanly enough doesn't really tell me anything, since your standards might be different from the pedal's required cleanness standard. Either it's clean enough to work with the unit, or it's not.
How does the unit track and sound when you play only one note, and let that note decay all the way? If you can pick one note, and the synth sounds fine through all but a long, sustained decaying note, then the issue is probably what you're trying to feed through it. Unless your playing is phenomenally clean (not just good enough), you're not going to get stellar results.
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Lastly... some folks just don't have the knack for playing with synth pedals. One has to play different from one's norm to make it work, and some folks can't think enough like a synth pedal to make it work. I get great results from all kinds of synth pedals, and others have posted about how those very same pedals have terrible tracking. The only conclusion is that we're both correct, and need only to add that "with the way that we play them" to clarify that it's not just the pedal in that effect chain which affects success.
Good luck!