Why is my answer accepted one day but not the next when reviewing and re-submitting the same answer?

https://teamtreehouse.com/library/objectoriented-swift-2/complex-data-structures/adding-instance-methods In the first task of this challenge it says:

"Given the struct below in the editor, we want to add a method that returns the person’s full name. Declare a method named fullName() that returns a string containing the person’s full name. Note: Make sure to allow for a space between the first and last name" with the given struct of:

struct Person { let firstName: String let lastName: String }

When adding in my code Sunday it looked like this:

struct Person { let firstName: String let lastName: String

func fullName() -> String { return "(firstName) (lastName)"

} }

I gave that answer on Sunday, and today when going over prior sections again, it is denied because it says I need to make sure I'm not adding any parameters to the function. In the code above I have the empty '()' so I thought that was good.

struct Person {
    let firstName: String
    let lastName: String

func fullName() -> String {
 return "\(firstName) \(lastName)"

}