Sticky post

Hi there,

i have a plugin on the website that shows post thumbnails. (recent post with thumbnail)
The plugin has the option to show a sticky post on top, but somehow it will not do this.
The owner of the plugin asked if my theme maybe interferes. He asked if there are any files with sticky features in the theme that can stop this from happening?

The website where the challenge is happening is:
http://www.zilvertron.com
And it is on the mainpage.

Thanks for your time,

Sjoerd