In Settings I had to change Automatic Update command to Full Calculate and then back again.
Once the original setting had been restored, the add-in ran as normal.
Copy the contents of the spreadsheet to a new one, and deleted the old one (it wasn't a particularly important spreadsheet, thankfully), and everything was fine.
I can only assume that somehow the file had become corrupted.
In a couple of recent articles I made a good effort to explain the basics of Excel VLOOKUP function to beginners and provide more complex VLOOKUP formula examples to power users. Because VLOOKUP is not the only lookup formula available in Excel, and its numerous limitations might prevent you from getting the desired result in many situations.
Somehow PI's Calculate method was no longer running during the normal workbook recalculate.
The Excel INDEX function returns a value in an array based on the row and column numbers you specify.
The syntax of the INDEX function is straightforward: If both row_num and column_num parameters are used, the INDEX function returns the value in the cell at the intersection of the specified row and column. However, when working with real data you would hardly ever know which row and column you want, that is why you need the help of the MATCH function.
I ran into an issue where some cells weren't calculating.
I checked all the normal things, like cell type, auto calculation, etc...