As with any garments that are waterproof that utilize a DWR coating, the waterproof coating is not always going to last. In extreme cases, your garment (and in this case, your bibs) may lose its ability to repel water and may begin to absorb it instead. Granted you keep them clean and avoid using any harsh chemicals when you wash them, the DWR coating should maintain most of its effectiveness as time moves on. If you believe that the performance has worsened, though, you can apply a spray-on DWR treatment to restore some of the effectiveness. When doing so, though, ensure the bibs are thoroughly cleaned beforehand.