I found issues like this all the time when using Web Essentials to compile SASS. In my opinion, SASS compilation has always been a bit flaky in Web Essentials. In fact, they decided to remove the compilation from Web Essentials 2015 (see 2015.0.2 in changelog )
I ended up uninstalling Web Essentials in favor of compiling with Grunt and using the Run Runner Explorer in Visual Studio to control the operation of my Grunt task. I found this a much better way to compile.
If you want to learn in detail how to implement this with Grunt and Task Runner Explorer, I wrote a post about it
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