That's one I have never seen before. By looking at the large geared wheel on top I see the different functions it goes through. Backwash, brine and rinse etc. If you put it in brine and rinse and remove the plastic tubing from the head, you should be able to feel vacuum at the fitting you removed the plastic tubing from. If not, that is the problem. It needs vacuum there to draw the brine from the tank. I can't tell you what to do next because I'm not at all familiar with that head, but the nozzle and venturi that actually create the vacuum from water pressure should be in that local area. If you can take that apart, look for a small nozzle looking thing with a hole diameter of a paper clip. That's where the gunk will be that is keeping it from working.
In a manner of speaking, the nozzle venturi should line up with the plastic tubing.
If you do have vacuum at that fitting, replace the plastic tubing. It probably has a pinhole somewhere that is sucking all the vacuum from the tubing.