Rotary Dumper dumps 3 empty cars

The module is built and assembled. The dumper works for a unit train.. 4 cars long :) Next is to load the coal : paint-dyed Perlite and hope it does not get too messy. As usual the video is on my channel. The cars sometimes get uncoupled and or derailed while dumping. With all the gates open the cars roll though the dumper ok and I put in different dummy couplers. The problem is the cars are NOT positioned correctly : the result seems to be the derailments and uncoupling.