Adding values in cells (stops adding after row 14)

I am trying to add numbers from cells if it IsNumeric and for some reason in column K the macro doesnt recognize numbers after row 14?
What am I not seeing?

The range column is "E4:E"
Search criteria is the letter "R" in column "E"
Then using OffSet, I go thru other columns and process data. Most of the macro works except for column K after row 14?

Option Explicit
Dim rShtRange As Range
Dim sShtLastRow As String
Dim vShtCell As Variant
Dim vRuntime As Variant
Dim vRunTotal As Variant
Dim vPartTotal As Variant

Sub TimeSheet()
If Left(ActiveWorkbook.Name, 2) = "TS" Then
    With ActiveSheet    'Loop thru Column C
        sShtLastRow = .Cells(Rows.Count, "E").End(xlUp).Row
        Set rShtRange = .Range("E4:E" & sShtLastRow)

        Range("G" & sShtLastRow).Value = ""
        Range("F" & sShtLastRow).Value = ""
        vRuntime = "0"
        vRunTotal = "0"
        vPartTotal = "0"

        For Each vShtCell In rShtRange
            If vShtCell = "R" Then
                vRuntime = vRuntime + 1
                If vShtCell.Offset(0, 2).Value <> "" Then
                    vRunTotal = vRunTotal + vShtCell.Offset(0, 2).Value
                    If IsNumeric(vShtCell.Offset(0, 6).Value) Then
                        vPartTotal = vPartTotal + vShtCell.Offset(0, 6).Value
                        MsgBox vPartTotal 'For testing
                    End If
                End If
            End If
        Next vShtCell
        vRuntime = vRuntime - 1
        Range("G" & sShtLastRow).Value = vRunTotal
        Range("F" & sShtLastRow).Value = ((vRunTotal / vRuntime) * 60)  / vPartTotal
    End With
End If
End Sub