Excel bug? VBA Application.Calculate not actually working

I have searched pretty hard for an answer, so hopefully this isn't a duplicate post.

I have a fairly calculation intensive model. The VBA isn't doing anything crazy, it just loops through 20,000 iterations by changing 2 values. The code is then copying the results to another sheet for each iteration (a 76x4 matrix).
My loop basically is as follows:

Turn off screen updating & go to manual calculations
loop start
Change value 1
Change value 2
application.calculate
copy values
next

Now here is the odd part...The application.calculate is not actually running randomly for many of the iterations. Therefore I'll have several result matrices with the same values, and then it will all the sudden kick back into calculation. The results matrices should never be the same. The ones which aren't calculating changes each time. I even tried doing application.calculateFull, but this doesn't work either. You can not replicate the problem when debugging. It's almost like excel is copying the results before the calculate finishes.

Please help!! I've tried so many things.
Thanks!

I mean yes, I do, but that shouldn't even have an impact on the values its copying during the loop.

I'm guessing this could even be an issue with limited hardware resources.

Still looking for help if possible.

I'm having the same problem. I have a small macro that opens an excel file, runs a data table, then copies values to another file. However, it does not allow enough time for the data table to calculate. I tried inserting the following:

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Application.Wait(Now + TimeValue("00:03:00"))

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yet it still doesn't calculate the data table.

any suggestions welcomed.