Make sure you have the Earth, High Beam and the Low beam feeds in the right orientation on the pins on the back of the light.
The Horizontal pin is your Lowbeam Filament, the Vertical pins are the High beam and the common, which in your case should be earth. If you get the Earth and The High beam wires cross over it will light up dull on Low beam as it back feeds through the high beam filament "looking" for an earth. High Beam will be full brightness whatever way you put them around!

