VBA multiple worksheets

I have three worksheets: X, Y, Z. Each has the same number of rows and columns. As each cell in Y and Z is created their average is put into X.

The code looks like:

For I = 1 to 8

   For J = 1 to 6

     Sheets("X").Cells(I, J).Value = (Sheets("Y").Cells(I, J).Value + Sheets("Z").Cells(I, J).Value) / 2

    Next J

Next I

even though the data in Z is there, it comes through as null. The values in Y are fine.