Mirena; Lengthy Periods

I've had the Mirena IUD for a little over a year, and my periods have become increasingly lighter and shorter over that time. However, several months ago my periods began to lengthen again. The flow is still very light, but I can sometimes bleed for close to two weeks. Two doctors have confirmed that the IUD is still in place, and my GYN doesn't have any advice aside from "you should wait it out".