Try pressing the alarm button, then leaving it a while so the light starts flashing (hopefully) indicating the alarm is armed. then open the door and see if the alarm sounds. Just trying to see if it's the alarm or the locking thats at fault. Sometimes mine arms the alarm ok but doesn't lock the doors, usually putting the key in the ignition and turning to MAR for a couple of seconds then turning it off again cures it.
If the alarm doesn't sound at all, is the light flashing rapidly? could be you have more than one fault, someone on here recently had the siren break (so the alarm was going off but no sound). If the light isn't flashing at all then the alarm isn't arming so it could be that the main alarm unit is at fault or possibly a fuse has gone.
The factory alarms can be a pain when they start playing up.
< Rob >
Spider 2.0 TS 1997
156 2.0 TS Selespeed 1999 (now sold)
147 2.0 TS Lusso 2002