Hi Noah,
She may keep coming up with an excuse whenever you ask her to come over because she is worried that the dynamics of the relationship has changed.
In asking her to become exclusive, she may feel that you have developed feelings for her that she is unable to reciprocate.
Even though you felt as though she was giving off a vibe that indicated that she wanted a relationship, she may have simply been caught up in the idea of it all.
In other words, she would just get a little lost in thought whenever she would hang out with you.
However, it is typically best, especially in a scenario such as yours in which the both of you were sleeping together without commitment, to let the woman be the one to strongly suggest getting into an exclusive relationship.
This way, you never run the risk of being too presumptuous with her and consequently ruin the casual relationship as a result.
As of now, this particular girl continues to text you first because she still likes the attention. However, she is worried that if she comes over and sleeps with you, your feelings for her will become even stronger.
The next time that you both chat, start a polite, open and honest conversation with her about why she is flaking on you.
Once you know her reasoning, you may be able to assuage whatever fears or doubts she may have and that may be enough to get her to start coming over again.