I had this problem about a month or two ago, and I thought something was wrong, but it really wasn't. Check your trunk latch...there's a tab that should be slid down, but it may be in the up position on your trunk. I didn't bother looking it up in the manual, but I'm pretty sure it's a security feature so that someone who is inside your car can't just open the back from the latch. Slide it down and it should work just fine.
Verdugo is right. I had the same problem right there at the dealer when I was buying the car. There is a safety feature latch that you can use so the trunk can not be opened from inside the car. Only with the key at the trunk.