TIMESTAMP field not automatically updating last_updated field

[Ferindo Middleton Jr]

I think I've seen this complaint posted before but I ignored but now I 
realize that in some of my db tables' last_updated field the value is 
automatically updating on UPDATEs to records while in other tables the 
last_updated fields for some strange reason aren't automatically updating.

I'll usually use the following line in my table declarations:

last_updated  TIMESTAMP DEFAULT CURRENT_TIMESTAMP NOT NULL,

In some tables it automatically updates on subsequent updates to the 
table and in others it will not. The purpose here is to have the 
last_updated field automatically append to the current timestamp... the 
application on the front end doesn't specify the time to MySQL but 
rather expects that it's always going to be UPDATEd to the current time 
slot.

What am I doing wrong what command should I issue to my tables to 
correct it? Thanks

Ferindo