I had a similar issue. I used a Dremel grinder to make the opening larger.
Thanks to both for your input.That is a very good question. Shovel handles are all over the map, diameter wise. The shovel that I have been using has a fiberglass shaft (from Lowes) on it, and it is too small. I ended up wrapping some black duct tape around the points of contact with the mount to make it fit correctly. So the issue goes both ways on that one. The optimal size diameter is between 1-1/8" and 1-3/8". The clamp will adjust within that parameter. The combo mounts that we have that hold both a shovel and an axe at the same time accommodate up to 1-1/2" diameter handles. Hope that helps.
Paul, thanks for the size recommendations, that definitely helps.