For Loop not completing

Hi All,

I am relatively new to macro and I was trying to use For loop in my code. It appears the loop is not running till end. My excel has some date columns but there are few columns where there is a gap in date between two consecutive columns. So what I was trying to do is to insert a column whereever there is a gap. and fill out the column header with the appropiate coulmn value. Below is the code that I wrote:

For i = 6 To lastcol
    colname1 = ColLetter(i)
    colname2 = ColLetter(i - 1)
    If (DateDiff("d", CDate(Range(colname2 & CStr(2)).Value), CDate(Range(colname1 & CStr(2)).Value)) > 1) Then
        For j = 1 To DateDiff("d", CDate(Range(colname2 & CStr(2)).Value), CDate(Range(colname1 & CStr(2)).Value)) - 1
            Columns(colname1).Select
            Selection.Insert Shift:=xlToRight
            nxtdt = DateAdd("d", 1, CDate(Range(colname2 & CStr(2)).Value))
            Range(colname1 & CStr(2)).Value = nxtdt
            Range(colname1 & CStr(2)).Select
            curcol = ActiveCell.Column
            colname1 = ColLetter(curcol + 1)
            colname2 = ColLetter(curcol)
            lastcol = lastcol + 1
            i = i + 1
        Next j
    End If

Next i
MsgBox i & " , " & j & " , " & lastcol

on executing the code the value that I am getting for i is 40 and of last col as 52 (lastcol is the last column of the datarange).

Will deeply apreciate if anyone can guide me on the mistake that I am making.

Thanks
-Som