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?
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.