When you feel like building on the positive moments, ask yourself "is what I am about to do going to bring us closer together, or push us further apart?"
This really helped me...whenever she pulled back, it was because I just HAD to send that last email, or make that last phone call, even though my conscience was telling me to let it hang, and leave her wanting more.
If she initially agreed to the weekend, then backed off, I'm thinking it's because she felt pressured; either by you suggesting the weekend, or something you said about it afterward. She may not feel you're ready for that step yet, and it is a big one.
My W is coming over to my (our) place for the weekend...so this is a big one for us, spending the weekend together. I will play it cool and make sure she feels no pressure at all. So far I think she is feeling comfortable...so I'm going to "stick with what is working" for us.
I'm by no means an expert, just saying what worked for me.