Instead of trying to pull the cam up, try pushing the plastic plunger down by jamming a large flat screwdriver under the cam and gently prying down on the plunger.
If that doesn't work, unscrew the plastic screw from underneath to relieve pressure. The slot in the plastic screw is very shallow, if you can't get it to turn there are two brute force methods. Heat up a large flat screwdriver with a torch and deepen the slot, then when if cools you can turn the screwdriver with a pipe wrench. Or, grab the exposed threads of the screw with a pair of vice grips and crank it out, obviously both of the brute force methods will damage the screw, but sometimes it's the only way.