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You really need a small digital multimeter for checking this out as there are multiple reasons this can happen. The most often cause is the automatic transfer switch. It might be a combination of those. Failing batteries is one we see all the time here. IF yours are over 3 years old that is a strong possibility. A meter is about $20 or so and available at most hardware stores and easy to use. That would make tracking down the problem(s) much faster. Also be sure that the main switch and battery disconnect switch are on so the batteries are in the loop. I will make a phone chat offer for you as some customers prefer this method here. All follow ups are included and fully expected.