Unable to initiate wi-fi sync from iPhone

I'm having a problem using wifi sync from my iPhone menus.

When I go to SETTINGS - GENERAL - iTUNES WIFI SYNC, I see the message that says I have to connect my iPhone to my computer using a wired cable and then check the box that says "Sync with this iPhone over wifi" on iTunes.

The problem is that my iPhone is already recognised on iTunes and I can wirelessly sync it using iTunes (since the above mentioned box is already checked), but it won't show the "Sync now" button on my iPhone to initiate the sync using the iPhone menus.

Does anyone has a clue how to solve this weird problem...?