Label filter on the Search results page is not displayed

On the search results page in our community, we have the following filters, but no Labels filter:

Here's the same filter on Lithium's community. Note the addition of the Labels filter:

Looking at the Search Page Thread Filters page (a subset of the Message Search page), it has the following XML which appears to include a reference to the search.widget.label-filter. I believe this is what builds the search filters at the top of that page. 

<quilt layout="single-row-full" nestable="false" disableTopCssClass="false">
  <add to="main-content">
    <component id="search.widget.location-filter" isDropDown="true"/>
    <component id="search.widget.label-filter" isDropDown="true"/>
    <component id="search.widget.author-filter" isDropDown="true"/>
    <component id="search.widget.post-date-filter" isDropDown="true"/>
    <component id="search.widget.bling-filter" isDropDown="true"/>
    <component id="search.widget.type-filter" isDropDown="true"/>
    <component id="search.widget.content-filter" isDropDown="true"/>
  </add>
</quilt>

Are there other options I'm missing to get that Labels filter to show up in our community?