Despite what others think, I actually believe there is a "holding" period for new sites. I built a brand new site back in February - added a ton of content and it was all indexed pretty quickly. Did no link-building or offsite seo at all and in the first two months, I had no organic keywords ranked. But in the third month, I got like 250+ organic keywords all at once (and improving), no link-building and no changes to the content. All the same. Ranking is still improving every day somehow just from the content, but in the beginning it was completely stagnant.
Content/site age seems to play a role IMO. How big or how small, I don't know. But I've seen many cases of sites gradually improving in serps as they age, with no links at all.