Problem using Loop Cut and Slide I'm following a tutorial for beginners and I can't figure out what I'm doing wrong despite a lot of time trying to figure it out.

When using the Loop Cut and Slide tool it is behaving differently for me than it does in the video I'm following.  I'll show the images here in a second to give you an example of what I mean.  So for me, after click-selecting my wanted axis, the tool selection only spans as far as the second next edge, where as for the instructor it spans around the entire wall - in this case I'm trying to mark a selection to extrude skirting.

The instructor using the tool:


How the tool behaves for me:


You can see that when creating the two horizontal edges (for the window) using this tool I had the same problem where the selection would only go as far as the second edge intersection it met.
The face at the right of the window is a NGon (a polygon with more than 4 vertices). NGons prevent Loop Cut to find his "way" in your topology, so LoopCut just stops when he found a NGon.