Just like when painting, problems can occur when staining wood. Premature fading, erosion, and peeling are three of the most common staining problems. Excessive exposure to the sun is one cause of fading stain. Most stains are resistant to fading - but they aren't immune to it. If fading is a chronic problem, painting the surface might be the solution. Another cause of fading is an uneven or thin application of the stain. Applying stain is different from applying paint in that lapped areas should be avoided if at all possible. Each board should be stained in its entirety to help assure a uniform finish.
Erosion of the stain is usually caused by severe exposure to the elements. Often less expensive stains don't hold up as well as higher quality ones. The only solution to eroding stain is to clean the surface and restain with a higher quality stain. Like fading, if erosion due to weather is a chronic problem, you might want to consider painting the surface instead of staining.
Stains rarely peel, but there are two reasons why they might - the stain is unable to penetrate the surface, or chemicals leach from the wood and push the stain off. Stains are different from paints in that they partly absorb into the surface. If stains aren't able to penetrate, they won't be able to adhere. The surface should be scraped and allowed to become porous. By sprinkling the surface with water, you can determine if the surface is porous enough to absorb the stain. Once it is, clean the surface and restain.
Chemicals or resins from the inside of the wood surface can bleed through the stain - causing discoloration in the affected area. Green or pressure treated wood is often the cause for this situation. The only solution for this problem is to allow the wood to weather until it's stable and then clean and apply the stain.