Tried switching them around - top clip to bottom clip?
I had this issue on my old victa commando, seemed tight, one day I stopped mowing due to rattling. I ended up holding it in pliers and hitting the clips tighter (these were new clips on a good used muffler). worked a bit but came off shortly later if memory serves.
Pretty sure I put a piece of small dia steel as a bushing around the head bolt. That fixed it, but it was a bit hard to get the clips on.
I agree - its doesn't make sense why it was tight and is now loose
