My guess is a defective switch. That would be where I would first look. Although unusual, they do go bad from time to time for various reasons. Always remove power from the switch before troubleshooting, by switching off the applicable circuit breaker first. NOTE: If you do not confidently posses the skill sets necessary to troubleshoot and replace if necessary, PLEASE consult with a licensed electrician.
I have the same exact problem. I will try the compressed air tomorrow do not want to run a new line under the concrete. The air bubbles blow so hard that it splashes water on the switch at the tub I bet it is causing my problem? Is there a way to adjust the amount of air pressure down from the blower?