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1) Dimming on lighting typically suggests a loose neutral wire connection or splice somewhere in the branch circuit and/or using the incorrect type of dimmer switch.
2) A faulty neutral connection can reside anywhere from the branch circuit neutral termination on the main panel neutral bus bar to any box on the circuit. The circuit can easily tandem thru other wall boxes (receptacle boxes, wall switch boxes or junction boxes
) before it reaches it's final destination.
3) What type of light bulbs are the can lights using and rated for? Incandescent, CFL
Does the light bulb wattage exceed the wattage rating of the can lighting fixture?
4) What type of dimmer switch is used? Is it rated for incandescent, CFL or LED's?
Incorrect type of dimmer switches can also cause a dimming problem. Need confirmation on the type of dimmer switch?
I suggest to temporarily install a regular ON/OFF snap switch in place of the dimmer switch to confirm if the lights still dim. If still dimming, then the problem is a loose neutral connection. If no more dimming, then the problem is attributed to using an incorrect dimmer switch and/or incompatible light bulbs. Proper dimmer switch types and bulb type compatibility are a must.