By now, you've ruled out storage and basic adjustments. The next suspect is your grinder. Humidity makes grounds stick to the burrs and chute, so clean it thoroughly. If you're using a blade grinder, consider upgrading—even a decent burr grinder will give you more control. Also, try single-dosing: weigh beans, grind, and use them immediately. This avoids stale grounds sitting in the hopper.

Still no luck? Check your water. If your tap water is hard or heavily chlorinated, it can dull flavors. Use filtered or bottled water with moderate mineral content.