I would guess the problem may be lack of weight over the back wheels and lots o weight at the front where it scrapes along the ground making a lot of resistance.
It may help to put some ball castors at the front to reduce the resistance and if you make it so the robot is slanted backwards then more weight may shift to the back? You could also possible move the fan motor further back or the wheels further forward.
Did you just use normal bike chain for power to the drum? Did you use 1 or 2 chains?