With no actual sass file to look at I assume that you are using features that aren't implemented in libsass yet. To quote from the offical node-sass repo (on which grunt-sass) is based on:
The libsass library is not currently at feature parity with the 3.2 Ruby Gem that most Sass users will use, and has little-to-no support for 3.3 syntax. While we try our best to maintain feature parity with libsass, we can not enable features that have not been implemented in libsass yet.