well it's not really at the discreption of those in the airport there some fact u need to consider if you wanna try to go sby on a previous flight. One of there is whether you sent any luggage if you're transit :in that case sby on previous flight is not allowed. second it depends on the availability of the flight consider that if the flight is overbooked priority goes to those booked on that flight but that for overbooking reason went sby. Third you need to consider your fare. Most of those fares don't allow you to get an earlier flight unless you pay a penalty starting from 50 euros, and some fares don't even allow you to change unless you buy a brand new tkt. Full fare and c fares usually allows you to do that.
Moreover your skyteam staus ,even if you're plus won't help you , as what i wrote before are the basic conditions to get you on an earlier flight.
Sometimes but it happens very rarely there is some deregulamentation for some reason and fares and not even considerated but as i told you some extraordinary factor has to come up in order to do that.

things don't happen at the discreption of someone....there are rules to follow even if you think alitalia is messed up hope i made it clear for you
bye bye