Lately I've been having trouble with trying to set up a day 2 with girls I've met. I never have a problem with opening and getting the number close, but I can't seem to keep their interest enough to meet again.
For example, recently I met this one girl at a caffe. The girl was working on a short story and I soon figured out she was a big comic/fantasy nerd, which I love. We had a pretty cool conversation about books, comics, anime, etc... I was able to convince her to come on an instant date to get some lunch. After ward I got her number and we both left on pretty friendly terms. I waited a day and a half before I texted her again, but she didn't respond. So I tired calling her and it went straight to voice mail. Any advice on how to prevent this in the future?

Ah cold numbers. Congrats on having no issues opening and approaching.

I am no PUA but this much I know: you need to plan a Day 2 together with her, and in front of her, right there and then, before you leave her or ask her number.You tell her in detail where and what you guys will be doing on that day and that its going to be awesome because you are SO excited to be visiting this place. It's about YOU having fun and she should be excited if you are excited.
If you live in a big city with lots of cultural happenings it should be easy. What is your passion? or do you both have a mutual passion? Say photography. Now, you happen to know that there is an excellent photographic exhibition at the central gallery in the city, or that there is this comic book fair convention, or there is a food market with great food stalls, and you guys are going to go there together. You explain to her how the weather is going to be like (even it you are just making it up) and you guys are going to meet at a specific spot and its going to be a lovely day. She needs to visualize this day with you in her mind. But don't over sell it!
You tell her next week its a crazy week but the following week you should have a free half day open during mid-week and that's when you are going to go. As you hug her goodbye you are going to ask her "So what are we doing this time next week"? And you get her to repeat the event you have both planned to go (make sure you drop a lot of "We's") - that is to make sure the day is nestled and registered in her mind and she has been paying attention to you.

Then, the weekend before that day you text her a reminder - and how you write that text in the best possible way I have no idea, hopefully a PUA can chime in on this.