Hi all hope you had as best a Christmas and New year as possible

Another Doh! moment I am afraid coming up, you know the little things you should remember but cannot for the life of you manage it
It is down to the ability to extend a web page, when building, down (or up ) in length by dragging the the control line as required to move the page background to the new length. Now it works fine in the main variant but seems to have disappeared in the mobile variant and whilst the line moves ok the page background will not shift.I have checked the lock/unlock on the layers but no matter what I do it refuses to release the page background for re-sizing
I know somewhere I have checked/unchecked a box some where but for the life of me cannot (After much searching and naughty words) find it

Help, someone put me out of my misery and tell me what I should know but have apparently forgotten

Ken