Hot water just turns itself off when using Alexa

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I use Alexa voice control to turn the hot water on, but I'm finding sometimes it will stay on for an hour or longer, other times it will randomly turn itself back off after only 10 mins. I don't have any schedules or time blocks set and I've tried it on both home and away modes and still have the problem. Hoping someone can help, I'm tired of having cold showers.