I have come up with an ugly workaround to this problem which seems to have been caused by removing the logo (using a filter as described elsewhere in the forum). If there’s no logo area, then the mobile menu doesn’t seem to be able to pop up. I added a tiny transparent logo which doesn’t actually change the look of my site – just a few empty pixels – and removed the filter so that it IS displayed. So now when the page is made mobile size, I get an empty logo area with a mobile menu at the side. The big white space is not very nice on a mobile device, I would really appreciate a better suggestion! Many thanks.