One of the belts sits a little tight new, I forget which one now, also you can get some binding on the spur shaft thing, it's got shims for the old version but comes with a new steel one (and it's a little different), so if you build it like the manual says that will bind a little, re-shim that and back off any belt which is too tight, and you should be golden.
It's just tiny little bits that cause a little grief on the Photon build, mainly shims etc and lack of them at times, mine is slop free and rolls free after early tweaks and now a few runs, built like a tank as well, hit another car at full tilt back straight last meet, my bumper is kind of trashed but could not find a single thing broken on the car, and I'm 'still' using alloy front shafts and plastic spool ends!.