I have two Blaze lights and 3 mounts which fit fine on 31.8 mm bars. The mount is adjustable and may not be set right in your case. On the base of the clamp under where the light mounts, there is a tab that engages notches in the band. You can push the band inward to make the strap shorter for smaller diameter bars, but you need to use a tool to lift the tab to be able to extend the band back out to fit 31.8mm bars. If you just yank the band out, you can strip the tab and it won't engage adequately anymore. Use a small screwdriver to lift the tab and pull the band forward/out until the tab can engage the last notch in the band. See tab engaged in last notch in pic below.




Also, position the movable friction pad on the band toward the front of the bars rather than underneath it. The toggle should be unscrewed most of the way to get it to engage in the slot of the band. Once engaged, it can be screwed in a bit before clamping if need be to reach the desired tension. Below is a pic with it as set after just removing it from 31.8mm bars.