You need to check with your voltmeter between ground and the contacts on the On-Off switch. With the switch ‘Off’ there should be 9volts on one side and nothing on the other. With the switch ‘On’ there should be 9 Volts on both sides.
Check that the power switch is not rubbing on the case. Sometimes the switch can appear to be latched ‘On’, but the contacts are not touching because the button has not latched properly (i.e. it has been pushed in and held by the case, but has not dropped back to the ‘Latched-on’ position. With the power button in the ‘On position, gently pull it out from the case.
If the switch is operating freely, but 9Volts does not appear on both sides of the switch when latched ‘On’, either the switch connections have a dry joint or the switch is faulty.