The trip data is transmitted to Progressive through a cell phone connection and has the same potential lose signal. They seem to allow for a few days of amnesty before they call.
I've had about 4 of Progressive's Snapshot (MyRate) devices plugged into my Buick over the last few years. I think because I've gone through so many, they no longer call me to reset it AND I still get the discount
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Also note - participation can only lower your rate.
(I'd link to the Progressive Snapshot FAQ page, but I'm a lurker.)