I have one of these vetus pumps supplying the house system on a live aboard 60fter and it has no issue getting sufficient pressure to the shower
on our second deck
The spare I bought is a seaflo brand which is identical apart from the box it came in, sticker on pump
and cheaper price
Seems obvious they come from same factory.
Perhaps its a pressure switch?
Usually a knock with a screwdriver can get them going until next time.
A spray with wd40 makes them work
A new one is better again and they are cheap
if you look online.