My problem is that the fabric softener doesn't

I know this may sound silly, but I have seen this problem solved on many occasions by cleaning the filter screen(s) in your cold water supply hose!
This is because the softener dispenser must fill with cold water (on top of your softener liquid) in order to start the siphoning off of the softener. If the cold water supply is restricted, the dispenser will not receive enough water begin the siphon effect.
Now before you tear things apart, try this test...
Begin a wash cycle by filling with HOT water, and observe (listen to) how fast the tub is filling for a few seconds... Now change the wash temperature to COLD, and notice the difference in water flow. If you detect a noticeable decrease in water flow... Clean the cold water hose screens!
How? Unplug the unit! Turn the cold water valve off, and remove the hose. (If you did the test above, you will easily feel which hose is the cold one.) Use a small brush and a squirt bottle to clear the screen inside the inlet to the washer, and also check the end of the hose where it attaches to the plumbing... some guys put a screen at both ends of the hose. Put it back together, turn the water on and CHECK for leaks. All OK? Plug 'er in and wash a load, and pat yourself on the back for saving yourself the price of a service call!
Hope this helps...
Bondo