Cool, you seem to have answered my
"friction puzzle"! (showing that two powered wheels cannot overcome the friction of two stationary wheels) with your two wheel drive set up.
Just FYI, it is really a bad idea to leave wheels unpowered this year, since you are wasting normal force on non powered wheels.
the friction each wheel can produce is proportional to the weight on that wheel, but if you divide the weight between four wheels and only power two, you half the available friction!
I'll be happy to clarify if I wasn't clear... let me know!
-Leav