tinynav three menu bars not showing

Hi, For some reason the three small horizontal bars on the right side of the main menu (to indicate it is a menu) do not appear, when the screen size is less then 960 wide (narrow window, portrait tablet or phone). The website is wickhall.org.uk I tried to inspect the element to diagnose whether it is just a color settings issue, but it seems that the bars are not there at all. Would you please be able to take a look and advise on how to get them back? Many Thanks Riccardo