Panasonic cells are sometimes narrower than other 18650 cells. The difference is small, but can have the effect of creating a looser connection in the terminal caps. Some Panasonic cells have been measured to be as narrower as 17.98 mm, and thus it can be helpful to make the cells slightly wider. This ensures a better fit in the terminal caps.
Two methods are commonly used to achieve this widening of Panasonic cells. Electrical tape can be wrapped around the terminals or 18650 heat shrink can be added on top of the existing heat shrink. In many cases, only the positive terminal is narrower on Panasonic cells, so it is recommended that you experiment with your cells, if you use Panasonic cells, to determine if you need both terminals wrapped or just the positive terminals.
Below are example photos of a battery pack made with Panasonic cells. Originally the pack builder found that the cells were too narrow and would be pushed back out of the cap by the spring force. Once he added a wrap of electrical tape around the ends of the cells, the caps fit perfectly and snugly. Another option would be to use sandwich plates, as described here.