Hmm, I think I forgot this when created your save file. I cheated 1000 citizens into existence, all 18 year old or so. But as they probably weren't very happy when they were in their childbearing age (as you probably, understandably, hadn't built a lot yet), they probably didn't get a lot of kids. These first 1000 people are probably dying right about now. Hopefully, your population should grow normally after that. If it doesn't, please Export your save file and send it to me and I'll fix it.
(sorry about this by the way!)