I built a much smaller plywood steam box a while back using that same Karcher unit.
I used it to bend oak, ash, pine and small branches, mostly very well, after steaming them for about 20 minutes each. (See oak test below). I was using 5-6mm strips about 400mm long. I'm guessing you're working with thicker stock? In this case you'll probably need to leave it for more than half an hour.
A few pointers for how you might get better results and reduce steaming time:
The internal volume is too large for that small steamer. (see picture for mine). Try steaming pieces in smaller batches, in a container that fits snugly around the pieces. Use small wedges along the strip to elevate the strips from the bottom, and get steam flowing around all its surfaces.
As mentioned by the comment above, the steam port is in the middle. Try running the steam parallel to the wood strips, by putting the port at one of those ends. I reckon the end grain exposed at either end is the most porous surface, and it should give you better flow too. I loosely sealed the other side to vent the flowing steam.
Also mentioned by a comment above, softwoods don't steam-bend very well. I found pine consistently failed alongside hardwoods like ash and oak. It tended to stay rigid, or break when bent sharply. Oak and ash are ideal woods. You want pieces with as straight a grain as possible.
You have to bend the material immediately after removing it from the steam box. It very quickly loses the ability to bend as it cools and dries.
I have never tried bending thicker stock, but you may have to laminate thinner pieces if you still can't get the results you're after.