Change Cells Formatting When Formula Result Changes

I am having some issues with my conditional formatting code. I need 5 condtions, so I cannot use the built in Cond. Format. Column E is the average of cells F,G,H and when I update the numbers in F/G/H the background color is changed to the appropriate color, but column E does not change color!!! Example: The average (E) will start as a 90 and have a red background, then when the inputs(F/G/H) are changed and the average(E) now equals 30 but it is still shaded red! Is there anyway to have every cell updated, to make the cell change from Red to Green automatically??? Below is a copy of my code.

Private Sub Worksheet_Change(ByVal Target As Range)
Dim WatchRange As Range
Dim CellVal As Integer
If Target.Cells.Count > 1 Then Exit Sub
CellVal = Target
Set WatchRange = Range("E6:H15")

If Not Intersect(Target, WatchRange) Is Nothing Then
Select Case CellVal

Case Empty
Target.Interior.ColorIndex = xlColorIndexNone
Case 0 To 19
Target.Interior.ColorIndex = 8
Case 20 To 39 ..........................