WP Smush always stop work in smushing.

Hello,

I used Smush Pro to one click compress all image, but I met some problems, always stop working, If stop work, I need to refresh page and re-click smushing, but when it dealt with a few pictures would stop working too, can you help me to fix it?

My hosting is AWS EC2..

Screenshot: http://i.imgur.com/D5iU42T.png

Thank you!