As a blind assistance, what I can suggest is to get the circuit schema that triggers the buzzer and although very unlikely, hope that there is some resistance in series that can be reduced, but the most obvious option would be to try replace the buzzer by another.
Another probable reason, can be the piezo be oscillating out of the frequency range for what was designed, and once again you should get more details about the circuit.
In any case, as a further attempt could also try to carefully replace the wires welded in piezo because probably the fact that are bulky and to be leaning on the device, may be dampening the vibration.