Our homeschooling team here in Blacksburg VA had the same mechanical problem. Aside from checking to make sure that your gears are not "pushed too hard" against the internal gear beam, our problem (now fixed) was two build related errors. Have two kids (separate from the original builders) go through the build step by step, counting hole offsets. This cured our problem.
Also.. here's a nice video of a working unit:
Coach of "Fire Breathing Rubber Duckies", FLL#19237
-- To UNSUBSCRIBE or CHANGE your settings, please visit https://listserv.jmu.edu/archives/vadcfll-l.html and select "Join or leave the list".
-- VADCFLL administrative announcements are sent via VADCFLL-ANNOUNCEMENTS-L. Visit https://listserv.jmu.edu/archives/vadcfll-ANNOUNCEMENTS-l.html to subscribe.