the most usual problem with this is the
...._regions_and_settlement_names.txt
file

there is a save type / format problem that means if you try and make a new file that looks like old one - or sometimes add bits to old one it doesn't recognise them.

as Aradan said work from old 'imperial_campaign_regions_and_settlement_names.txt'
make a copy of it and re-name it to what you want, then either change the ** bits within the existing
{**} **
format or copy and paste a chunk of those existing bits to the bottom of the file and then change the text within the ** in that area.... (it doesn't matter if you have more regions in that file than exist in the campaign)

I've never figured out why it does that but have had problems lots of times when trying to type in:
{new_thing} new thing

at the bottom or in a new file, unless I do the copy and paste of existing sections into old file.