Filtered index is not used when variable in WHERE condition
Filtered indexes can't use variables/parameters, unless you're building your query with dynamic SQL so that the query ends up getting executed with a literal.

A good article on the topic is Filtered Indexes and Dynamic SQL by Jeremiah Peschka. 

OR

As Martin suggested, you could add WITH (RECOMPILE) to the query, but that should not be used without understanding the potential repercussions (see RECOMPILE Hints and Execution Plan Caching).