The Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST) uses a large-format cross delay line (XDL) detector in its Far Ultraviolet (FUV) channel. While obtaining spectra, light falls non-uniformly on the detector due to the optical design and the spectral properties of the object being observed; in particular, bright emission lines from geocoronal Lyman-alpha can fall on the detector in more than 20 locations. As a result, some areas of the detector have received a much greater exposure than others. This non-uniform illumination has led to a timeand positiondependent change in the gain of the microchannel plates, which causes variations in the overall detector performance. We will discuss the effects of this gain sag on the science data, and discuss mitigation strategies which are being implemented in order to maximize the detector lifetime.