Some attributes are not displayed in the Filters List (Catalog > Navigation > Filters).

If particular attribites are not displayed in Filters list, these attributes are not enabled for Layered Navigation.

Solution:  Toenable a particular attribute you want to use in Layered Navigation, please go to:

Catalog->Attributes->Manage Attributes->{Click on the Attribute to Edit}->Frontend Properties->Use In Search Results Layered Navigation->Yes

See more details on Improved Layered Navigation page
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