You most probably have a JavaScript conflict – your wp theme or another plugin – which needs to be fixed. This codex article explains what this means and what to look for:

http://codex.simple-press.com/codex/faq/troubleshooting/what-is-this-jquery-conflict/

In fact I can see that your WP theme IS loading a second and invalid copy of jQuery.

While not producing errors, I see that the WP theme is also loading a copy of bootstrap.js which is unsupported by WordPress. The main problem with this library is that is stomps over some of the jQuery UI CSS rules which can make components mis-function. The key ios to get it loaded before jQuery and the jQuery UI core libraries which is not the case on your site at the moment.

You are right. I tried to switch the theme, and the issue goes away. 

I will follow your instruction later of the day to see if I can fix it.

thanks.

please let us know what you find…

The theme was using jQuery 2.0.3 along with jQuery ship with wordpress.
I removed the 2.0.3 version, and the Plupload panel show up; however, the start button is not working.

After further investigate, it seems there is conflict between bootstrap button js and jQuery UI button.
I took bootstrap button js out of picture, and now I can upload image without issue.

I guess this plugins is not bootstrap compatible. 

thanks for help.

Please see my comments regarding Bootstrap in post #2 above which explains the problem – and a potential solution. It really should not be used for WordPress based sites until such a time as the WP team properly support it.

the issue is that bootstrap and jquery ui (which wp already uses) clash…  if bootstrap is loaded after jquery ui, it stomps on it…  some theme others choose to use it because jquery ui is not always loaded on the front end…

wp is talking about adding bootstrap itself and it should ensure that it loads before jquery ui then…  until then, you can use them, just have to ensure that bootstrap is loaded before jquery ui…

I have the same issue. The pop up window does not show. How do I fix this?

same issue?  so did you fix the bootstrap issue causing it?

I’m having this same issue. I know it’s the bootstrap js that my theme is using. How do I ensure that bootstrap is loaded before jquery ui?

It depends on a few things – especially the WP theme in question. Have you done a search here on ‘bootsrap’ and the name of your WP theme? Some specific bootstrap topics do have code fixes in them so it might be a good place to start just in case.