replacing a light switch with a dimmer
The multimeter may be off trying to accurately measure an AC voltage that ends up looking like chopped AC cycles. 

There is a good possibility though that no load on a triac switch of the dimmer could look like an AC waveform of full voltage to the relatively high impedance of the multimeter input if the dimmer design has a resistor/capacitor snubber circuit across the triac. The diagram below shows the typical snubber configuration used (R2 and C2). R2 is commonly a resistor of a couple hundred ohms in series with the capacitor. When the triac is OFF the snubber circuit will be able to pass the AC waveform through when a meter with an typical input impedance of 20K to 50K ohms is used across the open lamp terminals.