Starting problems with the newer bikes are now well documented. Seems to be a common problem, although your bike is somewhat older.
My Griso suffers too. Actually, it is me who suffers. Push the starter and often nothing at all happens: sometimes it works after a few more prods. Sometimes it takes minutes, 10 minutes, or once it never started and I had to leave the bike. Ridiculous and infuriating.
The best explanation is voltage drop, on the newer bikes anyway.
I haven't checked the schematics but others say that Guzzi are still continuing with their practice of putting the full current up through the start switch. On the newer bikes, a heavier gauge wire down to the starter becomes a lighter thin wire. Significant voltage drop has been measured, as I understand it.
A solution would be to put in or parallel in a new wire of good gauge.
It could be the same thing with Antonio's bike.