Also 1 pin on the trimpot needs to be connected to both of the phototransistors....
**edited to add: as long as both flat sides of the phototransistors are facing the same direction. It looks like they are but It's hard for me to tell. **
actually.... it looks like your trimpot is not oriented right. Look at the two closely spaced pins on the trim pot - one pin each should go to the top side (opposite the flat side) of each phototransistor. The wiper (center pin) should be attached to the flat side of each phototransistor; shared. That should solve the problem. Sorry for all the edits haha!
You are only using the direct output of the right motors phototransistor. The resistor isn't really doing anything in this setup.