Was it working fine before for a good number of hours and now it is not, or are you troubleshooting a problem from first engine start?
An easy thing to check is to make sure the Sec Alternator is set to none (or to your secondary alternator pinout) in the configurator. If the 'Sec Alt' is set to something incorrectly, say your landing lights, it will turn off the alternator when you turn the lights on.
Also check, as Rob said, that the switch is not bad or loose switch wiring. I've had a bad switch problem too and it is easy to troubleshoot. Just look on your EFIS and it will tell you if the VP-X thinks the switch is on or off. It should match the physical state of your switch.

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Marc Ausman
RV-7 980 hours, IO-390, VP-X (sold)
RV-8 (flying a friend's)
Thinking about low and slow backcountry build.
VAF Advertiser -
Aircraft Wiring Guide
Book to help with experimental aircraft wiring.