Smudge tool not working

[steele]

I recently installed gimp to edit my photos. I end up having to blur a lot of licence plates with the smudge tool. however, every so often, the smudge tool stops smudging, and seems to do nothing instead. I was able to fix this issue a couple of times by resetting the smudge tool, but this time it still isn't working. The only setting I adjust on the smudge tool is the size of it.
Any advice would be much appreciated. For reference, I am using gimp 3.0