So, time to check the voltage at the bulb, with it connected and operating. If the voltage at your dim bulb is lower, you know it's an electrical problem and can work backwards from there. If the voltage is the same (as it should be), then the bulb itself is bad.
Likely you know this, but to access the bulb terminals, you pop off the trim ring, and then undo the three tiny screws that anchor the retaining ring around the bulb. You should be able to carefully let the bulb hang on the wires while you do the check.