social icons not showing

Hello,
As the topic name implies, we have problems in displaying the social icons on the secondary bar above the main menu.
(The site url is provided in the “private content” section of this message)
The steps we made are described as follows:
– we went on Theme options > Social profiles, selected the social icons we wanted (in this case facebook and Instagram), and added the proper url for each.
– then we went on Theme options > Header > Additional elements and selected in the header social icon dropdown the option “show on the left .in the upper bar”
– we clicked on the blue button “save all changes”
However, the result is that no icon can be seen in the upper bar of the frontend.
When pointing the mouse in the place where the icons are supposed to be, a colored square appears (with a tooltip displaying the related social media name), but no more than this.
Same result when changing the location on the header social icon dropdown of Theme options > Header > Additional elements.
Of course, we would like the true icons to be always fully visible.
Did we missed some configuration step and / or made something wrong? Just for the matter of clarity, we add that the linked social profiles are both existing and do not have any issue.
Thanks in advance of ny help you can give.