Sitemap is not refreshing

Dear support-team,

I have a problem with my Rankmath-Installation in WordPress. The sitemap is not refreshing even when a new post goes online. The last refresh was on 12.07.2020 although there were published two posts after that date. I have tried to refresh the sitemap by clicking save the permalink structure in the wordpress settings and by saving the sitemap in Rankmath, but there is no change. I have added the following code to the functions.php:

/**
* Filter if XML sitemap transient cache is enabled.
*
* @param boolean $unsigned Enable cache or not, defaults to true
*/
add_filter( ‘rank_math/sitemap/enable_caching’, ‘__return_true’);

But there is also no effect. What else I can do?

Greetings Kathrin