Hi zurc, usually this means you have an error in the css that isn't allowing the css to minifiy properly. Please look at the top of the bundled file for a message like "parse error at line xxxx". You'll need to trace the error to one of the css file lines.
Visual studio can automatically find issues in your css, so you can copy your bundled file into a "test.css" file, load that up in Visual Studio and check the Error List window to see if it spots errors in the css.
Alternatively, put it into a validator like this: https://jigsaw.w3.org/css-validator/