Thanks to everyone for the suggestions. I am using Kato Unitrack N Gauge and have about 25m running in a double-U layout; there are a mixture of Dapol and Bachmann/Farish locos and, after realising that I had posted this in the wrong forum, I also tested assorted Dapol locos and achieved the same result
The turnout is a Kato #4 and the traffic is running FACING (not shouting...), I tried using several spare turnouts and the result was the same. I also used several "Y" turnouts and the result was the same. I have about eight other #4 turnouts where the same locos traverse them FACING with no problems. The section of track is level and the traffic all follows the curved path and is integrated into a curved section of track (similar to other locations).
Eventually, I replaced the turnouts with a #6 and the locomotives now pass over it without fault. They also pass over two other sections that are identical but use #4 turnouts. I have a solution, but without clearly understanding the cause... as I noted, the #4 works perfectly in eight other locations, so... ?