You might want to check your dilution ratio for the yellow. For some reason yellow is one of the more difficult colors formulated into a detergent. In many cases the recommended dilution ratio is not the same as other colors or you may have to increase or decrease the dilution ratio to achieve the performance you are looking for.
If I remember correctly you have a LW 4000. Check that the injector isn't clogged. If it's ok, then the line should run out to the foam tube or the pod, whichever you have. You can take the line off the foamer for yellow and make sure it's getting that far. If it has the scrubbie pads to make it foam it could be clogged up and not letting the liquid through. My old LW had the scrub pads in the top center of the bridge and then tee'd out to each tube so if the pads are clogged up then nothing will come out of either side on that setup.
At injectors, switch yellow line with either blue or pink and see if the problem persists. If it does check foot valve, tip, or dilution in yellow bucket as mentioned, if problem moves to different color, check injector or line problem. I had same issue and a clogged injector was the problem. Good luck and Merry Christmas!
Just outo of curiosity, on the 3x foam manifold which port is yellow hooked up to? If you are looking at the pump stand head on, is it the furthest one to the left?