OK I'll try. This is just an "understanding" of mats.  Think of the mat as a place mat under your bowl of soup. Your butler placed the soup too far from you because you were eating caviar when he served it.  Seriously, now you want to move that soup closer to you. If you push the bowl, the place mat will stay where it sits. So you have to grab the place met, and drag both the mat and the bowl. It took me a while to get the hang of it too.
Here's the secret. "SELECT" the mat and not the bowl if you want to move it.

See screen snaps.
Showing the mat as being separate from the image
063 with the yellow mat is showing uneven border as you mentioned, and most important' look at the corners of the mat and you can see little red corner highlights. That is showing the MAT is selected. If you click on the image, then that is selected.
Think soup.
Hope this helps.
Fred
PS I assume you are familiar with borders as opposed to Mats for quick EVEN borders.

Hi Mike,
Thanks for your reply. I think I have it figured out...

So I load an image, fit it to my page etc. then I want to add a Mat, (not a border) so I right click on the image and choose Photo Mats (uneven borders) then choose Add Photo Mat.

I choose the width and color and position. But the mat goes to another page.  However this time I noticed on the mat dialogue window it says,  Note: use "Optimal or Optimal/spaced" placement to continue auto (re)arranging. 

I thought about this and realised that I was trying to insert a mat while I had Manual placement selected, so I changed this setting to Optimal and yep it works. It also works on Freehand...so learn new stuff every day!  It is easy to forget this stuff when you have not used the software for a while.

Sorry to waste your time.