Well if I had to guess I'd say you're not building enough attraction with these girls before getting their numbers, hence you exchange a couple of text messages then never hear from her again. As for texting itself, you seem to have a handle on it. I would suggest that the text always be something that entices the girl to respond, a joke, teasing, something like the "evil twin" opener, shit like that. Don't just be like "Hey what's up?". Or send her a text that brings up something y'all talked about or saw when you met.
Example: I met a girl through Facebook, we exchanged some messages, I went to get her screenname but she said she didn't have any messanger programs. I said, "Well do you have a phone number or do you still prefer to use smoke signals?" She joked that she did still and gave me her number. The first time I sent her a text I said, "Hey Pocahantas, get any interesting smoke signals lately?" She said, "Well...actually...

" and it went from there.