I can see that there are a lot of questions with this title, but unfortunately I wasn't able to find my problem there.
So, the problem.
I'm using the payed, standalone version of the generator and a cron job is set to auto generate on every 6 hours (/usr/local/bin/php /var/www/html/sitemap/runcrawl.php).
sitemap.xml file is set to 0666 and "data" dir is set to 0777.
So, the sitemap.xml was never updated after the cronjob was runned. It is sill updated after manual run of the generator. OK pretty standart situation.
The interest part is that on every 6 hours, new log file is generated in the "data" dir. If I open it it looks that everything went OK, but still, sitemap.xml remains the same.
Thanks for your help.