Also bear in mind - there are other reasons why a 13 year old might not want to come:
She wants to hang with her friends in the neighborhood
She doesn't want to be around the GF
She's afraid her mother will be hurt if she goes
She's afraid her mother will go out partying if she goes to your place
She's afraid she can't hide something at your place that she's hiding successfully from her mom (eating disorder, substance abuse, boyfriend sneaking in the bedroom at night - yes, I know she's 13, but you have to start thinking of these possibilities).