The type of adds you get will be based upon the keywords you use, along with the content, and value that you provide to the visitor. However, like Hojo pointed out if google, or the advertiser has devalued your website the smart pricing can take effect.
That is why it is always best to try, and create the website niche around what your vistors are seeking, and then adding in adsense as only part of the entire setup process.
You could also try building backlinks, building more social media presence, which in turn could help to give a boost on how your website is more helpful to the visitors, and thus could help with higher click thru rates on the adsense adds that will then start to appear. Another tip: pay closer attention to your on page and off page SEO elements. All this stuff is now more important to google than it was in the past when placing their adds on your websites.
Hope my answer helps