Spray some WD-40 into the horn. Sometimes they get corroded.
Good point. The horn should have a bezel on the rear with a centered screw holding it on with 2 wires coming out. Take the bell/bezel/chrome plated semi'circle off and measure the voltage across the horn's 2 input power studs, not wire terminals attached, the actual studs of the horn and see if you get 12 give or take volts when the horn button is depressed. If not, it could be a problem in your wiring or your switch. Easy to check the switch, just short across the terminals on the rear of the switch and see if the horn blows.
If you get 12v on the horn's studs, then see if you have access to any moving parts internally that could be sprayed. Other thing is rap on it with something hard while pressing the horn button. If it breaks free, exercise it a couple dozen times to possibly free up what was stuck/corroded/
