It turns out that infrared proximity sensors don’t actually work all that well with either transparent objects (like clear marbles) or round shiny objects (like spray painted marbles).  They really need something to reflect the UV light right back at the sensor, ideally something diffuse and flat.  In version 2 of the Multiplicator I think we would either replace the UV sensors with light sensors or something less prone to missing the marbles fall by, or by replacing the marbles with something more opaque and square, perhaps boxes made from acrylic.