It is difficult to know why because we don't know what version is your PSE and what is your operating system.
That aside, if your camera is fairly new then first try to update your software by going to:
Help >> Updates
If this doesn't solve then it may be that your camera is not supported by your PSE (mainly 10 and before) and so you may need to use DNG conversion utility. The latest version is 8.1 but unpaid MVPs here will tell you more about it.