Hello Maria,
This has actually always been locked down so that only the user who created the comment would be able to delete it, even at the system admin level. More information on this can be found in this article under the 'Delete an update or reply' heading. If the user who made the comment is not able to delete the comment and their access level allows them to, then please open a support case and we can take a closer look at this.
Hope this helps explain the behavior.