To make apps load faster, programs utilize a "cache" which stores a copy of the program (or add-in) on the users computer to access much faster on subsequent opens.

This however can cause version problems as Excel will serve outdated copies of the add-in.
 
The best way to make sure you have the most recent version is to clear the Office cache.

Clear the Office cache

If changes you've made do not seem to take effect, try clearing the Office cache on your computer. 

For Windows:

Delete the content of the folder %LOCALAPPDATA%\Microsoft\Office\16.0\Wef\.

For Mac:

Delete the content of the folder /Users/{your_name_on_the_device}/Library/Containers/com.Microsoft.OsfWebHost/Data/.

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