@daniantong,
The issue might related to different issues:
1) You have uploaded large number of high dimension images to your uploader and it takes time to get them all loaded.
2) You have uploaded a few super high resolution images to your uploader but your have server limitation on upload size and this causes the issue.
3) You have Javascript error on your site.
If your case is not the first two cases, press F12 and send us the displayed error. To reach the error, you will see a red cross within the opened pop-up, click on it and copy the message. In case of first two issues, remove the images, resize them and re-upload. In case of large number of images create separate directories for each image group using Add directory button. Thank you.
