That's because you've got two different objects going along that belt, the RF box and the generator object. The generator begins on the far left of the belt area in a tile with no parameters, so it doesn't generate anything. There's really no reason you'd want to use a generator there because all you're trying for is a belt speed, which I would just haul out MCE Event for and cut the hassle.
(Also, it probably moves too far to the left sometimes because you're moving the objects 127 pixels to the left before they're in the exact center of the tile.)

ETA: it doesn't matter what speed you give the belt events.