IPhone : Why Label Outlets cannot be connected to UITableViewCell Class

I create a table view cell subclass and set it as the class of the prototype.I want to add the outlets to that class and connect them.

But xcode do not respond.

And i try to add the outlets of button in the next time,it works.

I want to display a picture,it is pity that i don't have enough reputations.

I don't understand why this happen.Can somebody help me,Plesae!!