I struggled with tracking at first too, but have really gotten it going as I've dug further into the pedal.
Most importantly, I think @Jonny5bass has a strong point about aggressive attack. Tracking on the FI improved for me as I got used to settings that allowed me to play a little lighter. I set the input so that the red just starts to come on with some of my hardest plucked notes, then dial it back a bit.
Good tip on the compressor... I have yet to try my comp before the FI to get the cleanest bass signal I can. So far I've been quite happy with tracking, but switching order IS on the to-do list. Maybe that will help improve tracking even more. I've wanted to switch the order for a while anyway, to see if that also changes the sound of my octaver. Currently I have the compressor directly after the FI and have been quite happy.
Turning Note On/Off levels down helped quite a bit. Thanks to those that mentioned it here... I was fairly oblivious to that until I read people talking about it. Derp!
Unlike @Quatschmacher and @BurningSkies mention, I actually DO have (much) better luck with tracking by turning the tone/treble down. But I'm also using bright nickel rounds, so that may have something to do with my experience. I've also noticed that a good setup with minimal fret buzz and clank helps as well. I found this out when I did a neck adjustment on one of my main basses... (Not-so)Mysteriously, tracking improved!
I will also mention (and no offense meant to anyone) that even though you may think your technique is good/clean...... it may be that a little focus on this area could help. I thought I had good technique, but the more time I've spent with the FI for the 80's band I sub with, the more I've found that focusing on this helped tracking and helped my overall left hand technique to boot. Another key technique to focus on is good string muting. The Impact isn't polyphonic, so residual string noise from other strings can impact tracking as well. (Sidenote here: I do advocate having good technique of course, but a FretWrap can also be a huge help in muting.)
As with anything on Talkbass... These are just things that have worked for me, and your mileage may vary. We all have different gear and play styles, so different things may work better (or worse) for different people.
5sg.