Sublimerge won't work on scss file

I installed Sublime text 3 and Sublimerge, but for some reason it cannot compare the .scss file. I'm not sure what I did wrong because everything was by default. But if I copy and paste both files to the No tab to compare, it works.

Are there any settings I should change?

UPDATE

Here are my steps:

• Download Sublime text 3 for Mac

• Install package control

• Install Sublimerge Pro

• Open the .scss file from github reop local

• Download another repo file in .zip and change to another folder

• Open the same .scss file in the folked repository

• Compare View

• New window opens but no diff, although they are obviously different