If she had plans tomorrow night, or wasn't totally sure she's free, that would make it harder for her to reply. It's much better to just ask when they are free and get them to agree to hang out one of those days. It's easy for a person to feel pinned down to make a decision on the spot when someone else chooses the day for them to meet up, and it's way easier to not reply than to make that decision right on the spot like that.