Growing grass in shade is a challenge, even if the package says that the seed is good for shade. It may be that you don't really have enough sunlight for a lawn. Even shade-tolerant grasses need a fair amount of light. They're designed more for dappled light than for true shade. It also could be that your trees are so mature that they're competing with the grass for moisture, fertilizer, and light. Because you are having good luck with other plants, I think your growing conditions are good for shade plants but maybe not for grass. Ultimately, it may be that a lawn is impractical in this location. Try planting ground covers and/or using mulches.