Windows 7: Docx is missing from Default Programs\Set Associations

Using Win 7, 64 bit and Office 2012.

This problem occurs when trying to a docx file in Windows Explorer.

· Some docx files have the correct MS 2007 blue "W" icon, others show a plain document icon.
o If I double click on the plain doc icon, nothing happens.
o If I right click the plain icon and "Open With\Choose Default Program\Always use the selected program to open this kind of file" is grayed out so I cannot point it to MS Word.

Another symptom that may help in finding a solution is that, when I go to "Control Panel\Default Programs\Set Associations" docx does not appear (docm does).

Finally, when I open a file from within Word, that same duality of some files showing the W icon and others showing the plain icon appears. However, unlike in Windows Explorer, double clicking the plain icon will open the file within Word.

What is going on and how can I fix it?

Thanks for any help you can provide.

maestrogn