Modifying Item Keywords on iTunes U Public

Hello,

I've added keywords ( at the feed xml item level) to an existing collection item but when seraching using those new keywords I cannot find

the specific items.

I've double ckecked the xml feed at item level and it looks like the following

....

<itunes:keywords>NEW_KEYWORD_A, NEW_KEYWORD_B</itunes:keywords>

...

That item did not have any keyword before.

Our iTunes U Public is hosted on our server.

What can be the problem ?

Any help or suggestion are welcome.

Diego