I use a data validation list with streetnames. One day Flash fill works nice: a colourful suggestion, I click Enter - moving on. Next I open that same doc, and it doesn’t work; plus the button Flash fill is grey. I check the settings, that didn’t change. I consider changes in the database, but… they’re street names. I change them a few times a year - I’d have remembered. What else could cause the Flash fill function to turn itself off and turn grey?
O: Office 365, Dutch language. File just turned over 10Mb.
I copied the list of streetnames to a new file, copied the sheet with formulas and adjusted the formula; same result. Made a brand new file, few lines of data: everything worked just fine.