Cell does not always recalculate

Hi folks, my name is Myles and I am a s/w engineer from Christchurch in New Zealand's South Island.

I have an issue with a cell not updating and wondered if anyone could help. I have searched the forum and not found a solution that works.

Here is my function

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The idea is that you give it an index and a sheet name and it goes to the sheet, looks up the index in column A and returns the value from column B.

I am using the function in Sheet1 and I have the two columns of values in Sheet2.

When I update a value in Sheet2 column O it does not update the value that the function returns on Sheet1.

• Selecting the Sheet1 cell and pressing Enter does not work
• Selecting Sheet1 and pressing F9 does not work
• None of the various macros/functions that call Calculate seem to work
• Selecting the Sheet1 cell, pressing F2 then Enter DOES work

I'm a bit stuck for ideas now so any help would be much appreciated.