Completed constructing your bot and it worked fine..but after few dry runs you seem to encounter some problems in it. I will be discussing some common problems encountered and their fixes here.
i) Only one motor not working : It happens when there is a connection problem to the motor ( unless you have wasted your motor ). Just check if the connection terminal is soldered properly or not, or if a wire has come off the motor terminal.
ii) Two motors of a side not working : This happens when
a) The connection inside the remote box i.e. at the switch is broken. Check the particular DPDT which controlled the failed side of the bot.
b) The node where motors of one side are connected with the wire from the remote, might have come off. Check that junction point.
iii) Side Shaft / Johnson motor is not rotating even after connection to the motor is OK : This problem occurs ( provided your motors are fine ) if you screw your motors to the clamps excessively tightly. Check the motor shaft after loosening it from the clamp and supplying voltage accross it.