I have a product quite similar (The Karcher Window Vac)
It is actually an user error. The product must be placed upright for a few seconds during operation so the liquid can flow to the dirty water container.
If I use my Window Vac to suck large amounts of liquid (dry the counter, for example) and i don't flip it upright for like 2 seconds every few seconds depending on the amount of liquid I'm squeeging, it will do exactly the same.