You have set overlap header for single product pages. To change this behavior, navigate to the following header settings and disable “Make it overlap” option (http://prntscr.com/12wtp2k).
As was suggested in the previous reply the following options need to be disabled. Try to remove checkmark from “Make it overlap” option. If you already remove it and nothing changes, please, provide us your admin access, so we can log in and check this on your side.
I have disabled that option but nothing changed unfortunatly. I am going to share our website details in private with you so you can check it into more details.