You might notice that a powerseller suddenly isn't a powerseller! Why! Does this mean they have done something wrong! Probably not as the most common reason for losing your powerseller status is that you havenn't met your powerseller sales targets. For a bronze powerseller, this is either 100 sales a month or £750 of sales a month. All a seller needs to do is go on holiday for a while or be busy and not list the required amount of items and hey presto - no powerseller!
This has happened to me recently and I will probably get it back by Christmas. (once I get around to listing some items!)