Stepping through with F8 the code runs, stops in standard execution with F5 - excel

Question

I've written a macro which sums numbers, grouping by year and by month based on our projection model.
It runs from start to end when stepping through with F8.
It stops immediately in standard execution with F5.
The first trouble is
Runtime error 91
in
issmIndex = Range("A1:Z1").Find("ck.IssMon").Column 'issmIndex an integer
Originally I tried Application.WorksheetFunction.Match(...) but had the same problem: runs in debug, but not in execute (Error 1004 instead).
I considered it could have been an Excel version issue (the Match function has a different name in the Italian version). I switched to a more neutral Find, but still no luck.

Answer 1

When you have an error with a line that is a combination of several commands, try breaking it down into the individual steps.
For example, this works:
Sub findDemo()
Const toFind = "blah"
Dim rg As Range, f As Range
Set rg = Range("A2:C5")
Set f = rg.Find(toFind)
If f Is Nothing Then
Stop 'not found
Else
Debug.Print "found in column #" & f.Column
End If
End Sub
Also see the example in the documentation for Range.Find().

Answer 2

Welcome to SO. Sometimes Excel reads code faster than executing, so when reading a command there is a previous one not finished. IT's weird but it happens a lot if your code does a lot of stuff and calculus.
Besides, when debugging, every command line is executed before reading next one, so you cannot detect this just debugging.
So if your code runs perfect when debugged but errors if executed as normal, try to add the command DoEvents right before the problematic line. Something like this:
' your previous code
'
'
'
Doevents
issmIndex = Range("A1:Z1").Find("ck.IssMon").Column 'issmIndex an integer
'
'rest of your code
This commands forces Excel to make sure everything has been executed before reading. It's kind of like a checkpoint, something like make sure you've done everything before going to next line.
DoEvents
function