Thank you both. I knew it was an issue of this sort but I didn't know what.

Chuck

The waveform from the inverter may not be appropriate to a device
which expects a sinusoidal or near sinusoidal wave. Your inverter
which is probably intended for resistive loads (or loads such as a
TV in which conversion to a different level or levels of DC takes
place) may not provide a true sine wave and possibly even gives a
square wave which makes the dimmer simply turn on fully no matter
how you set it.

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Don Kelly
remove the X to answer
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Yes this is the reason. The inverter outputs a square wave. You could
try reducing the 12 volts supply to say 10 and see if the change in
output is enough. But you may have to watch for overheating the
inverter. Another way would be to filter the inverter output but you
will need to have some fairly big chokes to handle the load.

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Best Regards:
Baron.