Typically this would be an issue with the wireless router. Most likely the culprit is not the router itself but limitations to its signal due to Wi-Fi interference. Two major causes of this type of interference is other Wi-Fi networks and obstacles. (Think of a wireless router as a light bulb. Anything that comes between this light source and you would reduce the light's intensity and would be considered an obstacle.) For Wi-Fi, this can be walls, floors, ceilings, metal cabinets, sheet rock, etc.)
Sometimes just simply relocating the router will make a big difference in the overall coverage. However, when that is not enough, one option would be to extend your current AirPort with another. This is known as an extended wireless network.