Problem with Appearance > Menus section

Hi there,
Hoping you can help///something has gone wrong with the custom menu via 'Appearance' > 'Menus'.

We noticed several menu items missing but when the missing items are re-added and saved they are not visible when the page refreshes.

Also when the 'theme location' tab is selected this is also unchecked when the page refreshes.

Two custom menus are running on the website in question and strangely this issue is only affecting one of them.

So basically we are unable to amend our menu or select a theme location for the menu.

Many thanks in advance