This indicates the dimension fields in the Item Matrix are not set up exactly the same (case sensitive).
For example if you have one item setup as:
Color - Black / Size - Large
and another setup as:
Color - BLACK / Size - Large
the system will recognize that both items are spelled the same so it will not try to create a double matrix grid in the Purchase Order.
However, the entries are not in the same upper or lower case, so the associated cells will not show as editable in a Purchase Orders.
To fix this, go to Database | Items | Matrices and locate the affected item(s). Edit the dimension fields to use the same spelling for related entries. Example: If most entries are labeled Small and one is SMALL, change SMALL to Small.
This issue has been filed as a problem report with Microsoft to be addressed in a future update. Until then remember that matrix cells are Case Sensitive.
Update: RMS users running the current edition of RMS should not experience this problem.