Shopping Cart Class not work well

Hi All,

I am green in CI, and I tried to use 'cart' libraries recently.
I follow the instruction from user guide and I find that record is added into database.
But, it is nothing can retrieve as I use $this->cart->contents().

environment:
+ Apache 2.4.4
+ MySQL 5.5.32 (Community Server)
+ PHP 5.6.3 (VC11 X86 32bit thread safe) + PEAR
+ phpMyAdmin 4.0.4

Anything I missed? Please help.


Thanks