Many factors effect your sites ranking ability, age of domain name, originality of content and traffic flow all can play a big role in how your rankings are calculated.
How often your site is updates, internal linking structure, value of external links and structure of the site all play their roll in indexing and overall ranking ability. Below are three major reasons that affect the time it takes to see a lift your site in the SERP's.
Search engine spiders can be slow to return: The greater percentage of sites are not on the Google 'must crawl' list, a select few sites are crawled very regularly but is the exception rather than the rule. For most sites out there it can be a wait of several weeks if not three or four months before an entire site index is updated.
It is not only about your site: If you have recently started a linking strategy with maybe a focus on viral content designed to real in those precious links, 'link baiting'. A considerable time period is going to elapse before you really feel the impact of the campaign. All those new links littered about also have to be crawled and indexed, and again this can take an incredible amount of time.
It is a question of trust and patience is a virtue grasshopper: Let us assume the search directories have picked up on those valuable new links. Due to manipulative linking practises the algorithm waits to see just how credible these links are. It goes into our saying one good link is worth more than many. A link from a reputable source will stand the test of time, like a fine wine it will only get better with age. This is even more relevant for new domains if you have just popped up on the landscape you may have to wait a while before you can be trusted.