Well, the problem might be that your IES file does not define the shape of the light. The “Use light shape” option only works if the IES file includes the dimensions of the light.
The format might look the same but that’s just because it’s a bunch of numbers basically. I’ve found that some, but not all, of the Erco files have dimension information in them.
The information about the “shape” of the light should be on the first line of data for the IES, which you can edit on notepad or any other text editor. (in the one I’m looking at right now it’s just under a “tilt=none” line).
There’s a bunch of numbers, for the Erco files i’ve found the easiest way to find the dimension information is if you see negative numbers in this line, if that’s the case, just change them to a positive value. I think that the value preceding these numbers states the units it uses, 1 for feet and 2 for meters (I might and probably am wrong about this tho). Change it to something you consider “right” or play with these numbers until the shadow looks correct in rendering. There are 3 numbers total, x y z, for the dimensions. I usually leave the last one at 0.