Floating bar not fixed

Hi! I have a problem with WP Socializer wordpress plugin. I don’t undestand why, while I am scrolling the page, the floating bar seems fixed (as it should be) and when I stop to scroll the bar disappears and returns at the original position.
My website is http://www.scienceandtech.altervista.org.
Thank you very much for your help.

https://ww.wp.xz.cn/plugins/wp-socializer/