Thank you vkmast for that detailed explanation.

I have experimented with Autoripple but it does not really address my problem, which is that although my video plays properly and the time in seconds is displayed on the upper left of the screen, the vertical marker on the timeline down through the tracks stays in one place all the time and does not progress along the track while the video is played.

So if I start with the vertical marker at 00.15 on the time ruler it stays there despite the clock on the left showing time progressing. And I cannot drag the vertical marker to different places on the timeline.

I'm wondering if by this "marker" you are referring to you actually mean the cursor?
If so, see Options > Preferences for relevant options.

Also try pressing the backslash/pipe key, usually above the Enter key. This tells Vegas to resync the cursor and the playback head and then keep the cursor moving along with playback. There are many things you can do that stop this process, including scrolling the timeline or clicking on it, so the resync is often necessary.

Thanks for staying with me, vkmast and for your input, Chienworks. Sorry about my imprecise terminology - I did, in fact, mean the cursor. I have now discovered that when I start a new project, my cursor works as normal. The problem is when I load a .mp4 file into vegas and try to edit that - then the cursor cannot be persuaded to move, whatever I do.

I have had trouble with the cursor on other occasions - for example, when in a new project and I press the stop key, it doesn't stop at the point where I stop it but at some point previous to that.