Ian's as reliable as a genuine Swiss watch.

You definitely need the correct size (gauge) cable.
We have experienced faulty Hozelok stop connectors like used on the van ports. If you use a di inside the van, Hozlok coupling were/are the favourite to couple that into the system. We found that when coupling these together the male fitting only partial unseated the stop valve causing an obstruction.
It would appear that this is a fairly common occurrance from reading other posts over the years.
I would certainly check everything else is working fine before overiding the low voltage setting. Start at the pump outlet and work from there.
I also did experience the hose on the drum getting a kink in it which blocked the flow once.