Picture framing is a term that is used to describe what happens to the coating when it pulls to the edge of the part. On flat pieces, the edges will have a heavier buildup resulting in what looks like a picture frame (see Fig. 1, 2).

Figure 1, 2 (left and center): picture framing with a transparent blue powder; notice the dark areas towards the edges

Figure 2 (right): a heavy metallic panel shot with a fan tip

There are some powder types where picture framing is more common, fortunately, there are equipment and technique adjustments that can help reduce the likelihood that picture framing will occur.

Use the following techniques for basic troubleshooting in order to help resolve "picture framing":

Try using a round tip nozzle instead of a slotted or "fan" tip as this can help. Fan tips are great for covering a wide area but often times push more powder to the edges of the spray pattern, which can increase the risk of picture framing. Round style nozzles create a cloud pattern that have more even coverage.
If you do not have a round nozzle to switch to, try to keep the edge of the fan pattern away from the edge of the part. Instead, try shooting the edges of the part with the middle of the fan pattern.
It may also be necessary to reduce the dosage (air/powder) so that the powder builds more slowly. In extreme cases it may be necessary to apply two lighter coats instead of a single heavy coat.