Validate voltage at the socket and input side of the switch. If you're under 110 that is likely the problem. Also check for floating ground at the socket (voltage check socket ground to an earth ground with the switch on, off, and dimmed). ground should always be 0v.
I'm assuming you have tried a different bulb with with switch. If not, if could be a bad bulb that is not properly setting "0v reference" internally.