The newly generated .css by SASS now doesn't have comments to tell which file the css is from, what might be causing it?

The old generated .css file has

 /* line 27, ../../../app/sty开发者_如何学JAVAlesheets/partials/products/_summary_pane.sass */

which tells where the css (or sass) is originally from. (example: the above say it is from _summary_pane.sass)

and now all of a sudden, these lines are all gone. What might be the cause for it?

You need to set the following option:

Sass::Plugin.options[:debug_info] = true

Docs here.

This also may be of interest to you:

http://nex-3.com/posts/92-firesass-bridges-the-gap-between-sass-and-firebug