Not happened only once in current days, and both on my computer and other
users computer.

I have written a most simple Vlookup function like
=VLOOKUP(A2,Sheet2!A:B,2,FALSE), but the result is #N/A. I can find the
matched record in Sheet2, and in tools=options=Calcultion, setting is
Automatic. So according to my understanding it should work. And later I found
that If I want to activate my function, I have to double click the A2 cell,
no changes and press Entry.

I have more than 800 records, so to activate it one by one will be a heavy
job.

Do you have any idea about this?