You need a voltmeter for checking the available voltage to the switch and each bulb and then perform a voltage drop test to find the bad grounds, bad wires, connections or the switch. It is really fast to test this stuff with a meter and the only way to know if you have eliminated all the resistance in each circuit. There are many YouTube videos to show you how to test for available voltage and voltage drops. It is faster to learn how to use the meter than trying to fix an electrical problem without it. Test lights are not a substitute for a voltmeter. They do not tell you how much voltage or resistance you have.