[Plugin: Google XML Sitemaps] Unable to change frequency

Hi there,
It’s very important for me too change the frequency of articles on daily instead of 20% Monthly
http://www.scommessecasinoonline.it/sitemap.xml

I have tried to change this from the setting control panel/ frequency and priorities but it doesn’t pick up the changes.

What do I need to do?

Thanks

http://wordpress.org/extend/plugins/google-sitemap-generator/