I had this SAME exact problem, while Lloyd and Alex walked me through lots of troubleshooting, i just asked for replacement trough opto boards and that did the trick for me. easy to swap. When i was changing them out, i did notice that the sensor (looks like a little LED) on one side seemed to have its aim a bit "off", perhaps bending it a little bit could have fixed the issue? not sure, but i had already opted for replacement.