GIMP - round corners filter runs, but doesn't work

[HeatherK]

After following the great instructions on this site on how to round corners using GIMP (I'm in version 2.6.8), I've realised that for whatever reason, the round corners filter doesn't work on any of my images.

I'm using 1-layer images, with no alpha channel, and they're in RGB mode. I can select the round corner filter, and run it (usually with an edge radius of 20 and a blur radius of 15) - but then, nothing happens. The new image it produces is exactly the same as the old image. It doesn't matter if I select "work on copy" or let it run on the original - the script runs, and my image doesn't change.

I wanted to see if anyone else has had similar problems with the round corners filters - I know I can round my images using the rounded rectangle selection, but that's not nearly as simple as the round corners filter should be. I'm guessing that there's something simple I'm missing, but I don't know what it is.

Suggestions?

(Also, I've tried refreshing my script-fu console, but that doesn't fix it either.)

Thanks much!