Bundle virtual-paths should not be duplicated, otherwise the dupe will overwrite the contents of the previous entry. To add more than one file to a bundle, wrap them in the params in the Include method.

For example, in BundleConfig.cs RegisterBundles, this:

bundles.Add(new StyleBundle("~/bundles/css").Include("~/Content/xxx.less"));
bundles.Add(new StyleBundle("~/bundles/css").Include("~/Content/yyy.less"));

Needs to be this:

bundles.Add(new StyleBundle("~/bundles/css").Include(
                    "~/Content/xxx.less",
                    "~/Content/yyy.less"
                ));

Also, the bundle virtual path depth should be the same as the depth of the file you are referring to (4 in the example below). If you don’t do this, relative paths in your less/css will fail.

	bundles.Add(new LessBundle("~/Content/xxx/less/yyy").Include("~/Content/xxx/less/yyy.less"));