Confusion about Toolbox

I am new to Xojo but very experienced with the .Net environment. I’m finding the use of the toolbox confusing. In .Net, one double-clicks on the desired control or drags it onto the form being designed. However, when I do this with the Xojo line control, for example, the line appears as I draw it on the form but then immediately disappears. It seems very strange behaviour.
I do hope that I’m not wasting others’ time by asking a question that has been asked before but may I ask: has anyone a clue as to what I am doing incorrectly please?
Kind regards, Sean

Just click and drag the control over the window.

As for the line, if you do not need obliques, it is often much easier to replace it with a 1 pixel high or wide Rectangle.