First make sure that the wax solenoid is indeed on by holding a micro screwdriver to the solenoid while on and you should feel a very slight amount of magnetism.. I'm assuming since you have a problem in all bays that they are hooked up and work correctly but this takes five seconds to ensure your problem is not electrical in nature before you start taking stuff apart.
Sometimes in slow or closed down car washes the chemical lines that feed to the pump can get clogged. The easy hack is to attempt to suck the crap out of the lines by having someone in the bay hold the trigger down with the wax function on. Go to the pump stand and find the ball valve that controls the flow of water to that pump. When you close that valve off you will create additional suction on the wax line that can possibly clear the lines. Close the valve until the pump gets really noisy and leave it there for about ten seconds. Don't leave it like that for very long because the pump seals can be damaged if they overheat. Then, open the ball valve up until the pumps gets good flow and the excessive noise goes away. Go to teh bay and see if you can tell if the wax is flowing.
Another problem is that wax products can cause certain solenoid plunger seats to swell shut and not allow flow. Find the wax solenoid for a particular bay and disassemble the solenoid. You can compare to a known good solenoid plunger from a
soap solenoid to see if that is a difference. If the seat is swollen, you can take a fresh, sharp
razor blade and shave the material flush. Reassemble and again you may have to try sucking the goo as described above to clear the lines.