some site columns no longer showing up on my list form

[Victoria Symons]

I have a SharePoint list capturing contract renewal dates. It contains lots of columns. Several of my managed metadata columns are not showing up on the default list form. I can only add tags to these columns via the grid editor once I have created the item in the form.
I am not using content types. All columns are site columns and this was working previously. Any ideas?

[#2]

@Victoria Symons 

Have you used content types previously?  The scenario you describe does sound eerily similar what happens when site columns get added to libraries and not content types.

As a trouble shooting / comparative measure you could clone the library via list templates (if possible), copy it elsewhere and see if the same occurs in the copy.  You could also active content types via the list advanced settings and this should update the columns section in list settings to show which columns are attached to content types and which are pegged to the library.

[#3]

@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.