Proximities have a dome shaped detection field, it could be that in one direction it is seeing the gap of the toothed wheel but not in the other I had a similar experience where as a quick mod a toothed wheel was fitted to a conveyor drive so that we could position pallets with some accuracy, when we first fitted it, it seemed to work, however, we did find that when reversing the conveyor we got varying results in the position (need to add that the system measured the pallet length, detected types of runners & blocks so that the system could run the pallet into the strapper, decide using PE's if the strap swords could pass through the pallet and move it back/forward to find the gaps between the blocks). It turned out that the "Dome" of the sensing field appeared to be mis-shaped in other words When going forward the leading edge of the sensor was far enough away that the field was not strong enough to trigger the sensor, but running backwards the sensor field would detect the preceding tooth. We tried adjusting the distance but this gave inconsistent results in both directions, after trying various sixed Proximities we ended up having the teeth on the wheel ground to a smaller width, this cured our problem. A little hard to explain but when the leading edge of the proximity came in view of the next tooth the trailing tooth was on the trailing edge of the prox, this on occasion gave no change in state, it was impossible to reduce the number of teeth to increase the gap between so the sensor was clear of both teeth, so reducing the size of the teeth made enough gap so the sensor would not stay triggered. Ideally, a sensor diameter smaller would have done the trick but we could not get one. We did try different sensors and some seem to work better than others, for future machines they used fork type sensors but because of the nature of large conveyors & tolerances on the drive these were only partially successful (often the fork would touch the toothed wheel & damage the sensor). Before anyone says it, yes it was mainly due to pallets getting stuck & hairy A%$% staff jemmying them to get them moving that caused the brackets to get bent.