Sounds as though you only have 1 horizontal thingy on the top. This is where your membrane/s is/are.
To start with you need to identify what your waste to pure ratio is. With the unit running you need about a 3:1 ratio - ie 3 litres of waste water to 1 litre of pure.
You have a flush valve - does it also incorporate a restrictor or some method of throttling/restricting the waste. The water needs pressure to work the membranes properly.
This is what we have.
http://www.ro-man.com/shop/index.php?route=product/product&filter_name=flushkit&product_id=308There are several different membranes that fit into these housing that produce different amounts of water, 36gpd, 50gpd, 75gpd, 100gpd and 150gpd.
The RoMan unit we have has 3 membrane housings with 3 x 150GPD membranes. We found this just sufficient to produce enough water to keep 2 of us going with seperate vans every day. We process water into a 1000 IBC tank and draw what we require.
Since October last year, we have a 3rd part time member in our team so water usuage must now be carefully planned. In winter the problem is worse as the tap water is colder.
Spruce