JavaScript loading but not being applied in Drupal 8 theme
The dumbest of all possible mistakes was the answer. I had a &lt;div&gt; wrapping my insertion of the page block, like so:

{% if page.slideshow %}
{{ attach_library('themename/zaccordion') }}
    &lt;div class="col-xs-12" id="slideshow"&gt;
        {{ page.slideshow }}
    &lt;/div&gt;
{% endif %}


I thought if I put the attach_library line there it would load it 'closer' to the actual code needing it (don't try to understand what I was thinking). At any rate, I noticed that I have an id="slideshow" here as well. So I had TWO id="slideshow" - one on this &lt;div&gt; and another on the &lt;ul&gt; in the block...that was causing the slideshow not to work. So I just changed the id of this &lt;div&gt; to something else and it worked.