Loading data from folder not loading all columns

Update: I have tested by creating a completely new Power BI model. 

I then created a custom function to process the files but the error remained when using the first file as the sample file. Starting over and taking each file in turn as the sample file, the error eventually disappeared when using the third file in the folder. Why this is so, is still an enigma to me, since all first three files in that folder have identical layouts, column titles, and data formats in each column.