some site columns no longer showing up on my list form

Victoria Symons 

Are you using modern experience?  If so, you should be able to add these hidden columns by...

Step 1 - Click on the down area on the "+Add column" on the far right side of the list

Step 2 - Click on "Show/hide columns" 

Step 3 - click on any hidden column, then close.  You should now see it in your list.