As the sensing distance of your sensors increases, the distinction between black and white surfaces starts to decrease. This problem can be exacerbated by infrared components of ambient lighting. The QTR-8x PCB was designed to help shield the sensors from ambient IR, but this may not be sufficient for areas with unusually high ambient IR or in cases where you need to mount your QTR-8x higher than the recommended 0.25" maximum. In these situations, you might fight it helpful to use shielding (e.g. a skirt around your robot) to block ambient IR.
When troubleshooting your QTR-8x, a digital camera can be useful in determining whether the IR LEDs are functioning as expected. Many older/cheaper digital cameras lack IR filters (most cell phone cameras fit in this category) and can therefore be used to determine if the LEDs are on or off. However, some cameras (usually more expensive ones) have better filters that block IR. In the picture below, a point-and-shoot camera shows the IR LEDs of our IR Beacon shining (in purple) but the DSLR with which the picture is taken does not show the LEDs.