Have you examined the drain pan itself? Perhaps it is rusted and has a hole causing the leak. If there is a hole, the water could leak out before it reaches the drain.

Does the unit turn off if the float is lifted/activated manually? If so, see above. If not, jump back on the servicer.

It is the drain line that clogs up with biological growth. But the switch never gets wet even while it is leaking. I can manually push the switch up and it shuts the unit off. But the water from the drain pan never makes it to the switch to turn it off and nothing clogging it. The unit is relatively new. Has a plastic drain pan. Service guys can't figure it out. Here is a pic. You can see the switch on the right and the drain line on the left. I am posting here for inspiration on why the switch is not getting water while the unit floods. I am wondering if the drain pan is not level.

Call a different HVAC contractor. This should be simple.

Is this an air handler? If it is you may have negative air pressure forcing the condensate back away from the float switch. You may need to install a P trap to allow the condensate to drain out of the pan.

Here is a video I posted last week: How the air conditioner P-trap works This may solve your problem.

You may be able to activate the switch by hand but it may still be too strong for the water level to operate it. Have you tried to make it "float" off with water?