jQuery slider not updating unless button is clicked twice

jQuery slider not updating unless button is clicked twice

I am using a jQuery UI slider, pulling values from an XML file and updating the slider.  For some reason, the slider only updates if I click the necessary button twice.  I threw an alert right before the function to update the sliders is called - the values are there, but the slider simply won't update the first time.  Calling the function multiple times doesn't change anything.

...
    document.getElementById("vidToEdit").innerHTML = file;
    document.getElementById("startTimeInput").value = sequenceStartTime[file];   
    document.getElementById("endTimeInput").value = sequenceStartTime[file]*1 + sequenceDuration[file]*1;
   
    alert(slideValue1 + " " + slideValue2)
    slideValue1 = document.getElementById("startTimeInput").value * 10;
    slideValue2 = document.getElementById("endTimeInput").value * 10;
   
    updateSlider();
...

function updateSlider()
{
    $('#sliderClip').slider("values", 0, slideValue1);
    $('#sliderClip').slider("values", 1, slideValue2);
}

You can see it in action here: http://www.spacebarstudios411.com/easyclip/create.php