The tool only works on slots/tabs with 4 nodes that are in effect creating a rectangle. What Bernd showed you is that you need to add the 4th node to create the virtual rectangle at those locations. Just adding the extra nodes would not work in this case though because of the extra geometry on the corners. You need to also remove the extra lines.
If nothing works, please post an example or the file itself here, it’s always much easier and faster to find the error instead of discussing back and forth.
I didn’t catch that it was an SVG file, I was expecting a lbrn2, my mistake, sorry.
But, when I do what Tim has suggested, i.e. remove the small errors in the drawing that prevent a resize from being possible, specify the exact old height, or expand the tolerance… and by adding 2 extra nodes, you get what you want.
You must remove the odd shapes and insert the extra nodes to line up with the bottom of the tabs for the end parts to work.
You have to know the size of the slots before opening the resize tool. In this case, 2.79mm. I chose 2.800 for old material thickness and 0.100 tolerance.