Pan slider has disappeared [SOLVED]

Hi there, hope someone can help here.

I use 2.1.2 version with Windows 10.
I was working on a mono voice track yesterday and now i come to edit it today, and have found that the pan-slider has vanished, and instead of it playing in stero with balanced L and R volume, it is jumping from either playing just one side, or playing both L & R but at different volumes!

The other editing tracks in the file have pan sliders, just not my main track! The only difference I can see from the others, is that I am not given the option to convert it to a stero track, whereas the others still have this option.

I cannot find anything on vanishing pan sliders, so presume they are always meant to be there. Help!

Adjusting the height of the track can make the pan-slider disappear …
Pan slider disappears.gif

Thank you Trebor!! So simple; I feel a complete fool for not even trying this. You live and learn.
Thanks again for this witchcraft.