Cannot get formulas to pull info from other Workbooks

I have three excel files that must "talk" to each other to function. They are very large with lots features. In order to work they must have formulas that reach into other. An example would be "=[OtherFile.xlsm]Sheet1!A1". This "OtherFile" is in the same folder as the current Spreadsheet.

For the longest time, this was done by clicking on the cell in the OtherFile, but that isn't working anymore and I have no idea why. If a click on a cell in "ThisFile" and type in "=" and then click on a cell in "OtherFile", nothing happens. It only places a "=" in the cell in "ThisFile" when earlier it would place "=[OtherFile]Sheet!XX##". If anyone knows why it doesn't do that any more, it would be very greatly appreciated.

Thank you.

Windows 10 Enterprise, running Microsoft Office 365 from 2016.