Excel formula stopped working

Hi All
I have an Excel spreadsheet that contains 2 worksheets .

It was created about 10 years ago, and at the time I created a simple formula in both sheets that simply creates a sum result when adding or subtracting numbers as I go along.

So, the formula is within 3 adjacent columns, first column is for a subtraction entry, the second for addition entry, and the third will be the sum of either entry resulting in a rolling total.

This is the same for both worksheets.

For some reason one of the worksheets has stopped calculating the formula, whilst the other is working normally.

Nothing has changed as far as inputting the usual data, so am not sure why one of the worksheets cannot automatically calculate the formula within the same open file.

I also notice that the rolling total which normally runs down through the entire "unused" column for future entries that are within the working worksheet, is showing only about 20 cells worth of calculations left (if that makes sense), whereas the non working formula worksheet has no rolling totals left, which is why it cannot automatically continue, so presumable the working sheet will also stop working once it has reached the next 20 entries.

Is there a limit within the spreadsheet as to how many rolling calculations you can have bearing in mind my file has been growing since 10 years ago? I am at 3,500 or so rows currently.

I have checked the Calculations tab, and this is set to Automatic. 
If I run the Error check, nothing comes back showing any errors.

So for now I have to manually enter the formula to continue the calculation, but only in one of the worksheets within the same spreadsheet file.

I have searched through various forums and help guides, and most point to the Calculation not set to automatic, whilst others suggest editing the Windows Registry etc...

Any help or advice would be appreciated.

Thanks