The reason it is only working for your scss files is because the Ruby HAML highlighting settings overrides your sass highlighting.
Goto Preferences > Browse Packages...
Find and open Ruby Haml.tmLanguage inside the Rails folder
change the fileTypes from:
fileTypes
haml
sass
foldingStartMarker
^\s*([-%#\:\.\w\=].*)\s$
foldingStopMarker
^\s*$
...
to:
fileTypes
haml
foldingStartMarker
^\s*([-%#\:\.\w\=].*)\s$
foldingStopMarker
^\s*$
...
Now the SASS highlighting package you installed should be working properly.