Change your Filter Regularly
The older your filter, the more contaminants it's holding. "Unloading" is a process that eventually occurs in every filter, and once the saturation point is reached, the filter will actually begin to allow small amounts of contaminants back into the water stream.
Since many contaminants are tasteless and odorless, you would never know that this process is occurring. For any water filter, always make sure you follow the instructions and change the filters regularly.