As you have already discovered for yourself, mechanical/technical issues are not usually covered by the standard defence of 'extraordinary circumstances'.

In practice, most (or all) airlines continue to just deny your claim. The only way to ensure that they take your claim seriously is if you take them to small claims court or equivalent.

Before doing so you might like to give them a final opportunity to pay up what is due, so sending some form of Notice letter before action laying out your claim in full and including the correct references to the precedent case law which impacts your claim (Wallentin-Hermann and Sturgeon). It may also help to check whether you have access to legal expenses cover and let a lawyer pursue the claim on your behalf.

You don't say where you reside but insofar as EU law is concerned, you can bring your case either in Italy or in NYC. The latter may need the local court to decide whether they wish to hear a case involving external EU law but has been done successfully in the past by this poster:

http://www.flightmole.com/forum/showthread.php?t=779

Good luck with whatever you decide