Pull your dimmer switch off and check for corrosion along the edges of the spring. I've had two of them completely caked with the green and white crusty stuff.
An old toothbrush and some WD-40 is usually all you need to clean them up. Wipe a little dielectric grease on the contact points with a Q-tip, plug it in and try it out.
Almost all along inside of that spring is white crusty stuff then on one side where it clicks for the dome like that spring is actually burnt/broken so think better route just buy new switch.