Try checking your web page's code with the W3C Markup Validation service and fix any HTML errors reported.

Also, try upgrading your galleries to the latest version of Juicebox (v1.3.3).
Instructions on how to get the latest version and how to uphgrade existing galleries can be found on the Upgrading Juicebox support page.

The above suggestions may or may not help but if any HTML errors are fixed and your galleries use the current version of Juicebox, then we can eliminate these factors from the problem.

The problem may be related to the browser timing out whilst fetching the images from Flickr's servers. As a test, you could try loading local galleries (with images stored on your web server and listed in the gallery XML files) to see if this makes a difference.

The problem is certainly strange and I do not know the exact cause.
However, as I mentioned above, if the code on your web page validates correctly and your gallery uses the most recent version of Juicebox, then we can at least rule out HTML errors and previous Juicebox bugs as possible causes.

This should not make a difference. Large images will take longer to load but they should not cause a gallery to not display. The preloader (spinner logo) should be visible whilst an image is loading.

You could certainly try. (The more information we have, the better.)
I suggested using local galleries rather than Flickr in order to try to determine whether Flickr was a contributing factor in the problem.

Juicebox does not have built-in support for Picasa Web Albums.
If you would like to display a Picasa Web Album as a Juicebox gallery, you could adapt the SimpleViewer for Picasa Web Albums script from this web page.

Essentially you would use a PHP script to dynamically create the XML data on-the-fly as the gallery is being displayed.
You would point towards this PHP script using the configURL configuration option in your gallery's embedding code.

For example, use the following code in a file named 'config.php' (changing the Picasa Username and Web Album Name as required) and use configURL: 'config.php' in your gallery's embedding code.

<?php

// Simpleviewer for Picasa Web ALbums
// get all the details on www.paulvanroekel.nl/picasawebalbums/


    $userid = 'picasateam';
    $album = 'VegasWeekend';
    
    
    // build feed URL
     $feedURL = "http://picasaweb.google.com/data/feed/api/user/$userid/album/$album?imgmax=912";
     // $feedURL = "http://picasaweb.google.com/data/feed/api/user/$userid?kind=photo";


    // read feed into SimpleXML object
    $sxml = simplexml_load_file($feedURL);
    
    // customize your gallery
    echo "<juiceboxgallery galleryTitle=\"$album\" >\n"; 


    // iterate over entries in album
    // print each entry's title, size, dimensions, tags, and thumbnail image
    foreach ($sxml->entry as $entry) {
      $title = $entry->title;
      $summary = $entry->summary;
      
      $gphoto = $entry->children('http://schemas.google.com/photos/2007');
      $size = $gphoto->size;
      $height = $gphoto->height;
      $width = $gphoto->width;
      
      $media = $entry->children('http://search.yahoo.com/mrss/');
      $thumbnail = $media->group->thumbnail[1];
      $content = $media->group->content;
      $tags = $media->group->keywords;
  
          


        echo "<image imageURL=\"";
        // url van photo
        echo $content->attributes()->{'url'};
        echo "\" thumbURL=\"";
        // url van thumb
        echo $thumbnail->attributes()->{'url'};
        echo "\">";
        echo "<title>";
        echo $title ;
        echo "</title>";
        echo "<caption>";
        echo $summary ;
        echo "</caption>";
        echo "</image>";
        echo "\n";

   
        }
        

    echo "</juiceboxgallery>";

?>