Excel VBA Code Doesn't Work Properly in Group - excel

Question

I am new to VBA coding just like most of the people who ask questions like this one here. :)
I wrote a code which runs without any error but for some reason the second part is not executed if it is run in a group. Surprisingly, it does its job if I run the second part of the code on its own.
Can someone give me a bit of guidance what is causing this problem?
Sub Macro1()
Endrow1 = Cells(Rows.Count, 1).End(xlUp).Row
Dim j As Integer
Dim k As Integer
For i = 3 To Endrow1
With Sheets("Sheet1").Cells(i, 1)
If Cells(i, 1).Value = "No" Then Cells(i, 1).Value = ""
If Left(.Value, 3) = "Yes" Then
Rows(i).Font.Bold = True
a = Cells(i, 3).Value
For j = 0 To 30
Cells(j + i, 12).Value = a
Next j
End If
End With
Next i
'------------------------------------------------------------------- Second Part:
For j = 3 To 300
With Sheets("Sheet1").Cells(j, 7)
If Cells(j, 7).Value >= a And Cells(j, 7) <= Cells(1, 11) Then
If Cells(j, 12) = Cells(j, 12) Then
On Error Resume Next
For k = 0 To 30
If Cells(j + k, 12) = Cells(j, 12) Then Cells(j + k, 12).Interior.Color = 255
If Cells(j - k, 12) = Cells(j, 12) Then Cells(j - k, 12).Interior.Color = 255
Next k
End If
End If
End With
Next j
End Sub

Answer 1

When you debug this, do you get past this condition "If Cells(j, 7).Value >= a And Cells(j, 7) <= Cells(1, 11) Then"? Also, what is the 'on error resume next' hiding? Lastly, I don't think your 'With' structure is doing anything.
Thank you for your answer Martin. The problem was caused by my first if statement in the second part.
This is the correct version:
For j = 3 To 300
With Sheets("Sheet1").Cells(j, 7)
If Cells(j, 7).Value <= Cells(1, 11) Then
If Cells(j, 12) = Cells(j, 12) Then
On Error Resume Next
For k = 0 To 30
If Cells(j + k, 12) = Cells(j, 12) Then Cells(j + k, 12).Interior.Color = 255
If Cells(j - k, 12) = Cells(j, 12) Then Cells(j - k, 12).Interior.Color = 255
Next k
End If
End If
End With
Next j
I will get rid off the with statement later. :)