When buying traffic from any partner (e.g. Google, Bing, Yahoo, or even intext or email companies like Miva, Adconion, etc.) there is one thing you supply to them that is uniquely yours: the destination URL. Social updates, tweets, and emails that contain links to our destination sites also contain URLs that we provide as the originators of those updates we're sharing with our customers. This URL contains the parameters (if you choose to include them) that make it possible to later analyze the performance of your campaigns and adgroups in analytics. Many times, even when we think we've provided URLs with parameters to our traffic partners, or in a Tweet or Facebook share, we don't sometimes see the results in Analytics. Please let me know how often you've seen this issue, and what impact it may be causing.
I've seen this before. Google Analytics is not perfect, may I recommend the likes of https://www.woopra.com, https://mixpanel.com/ and http://clicky.com/
The problem can arise when the parameters aren't able to pass through to the analytics solution - errors, cookies and such.
Are you experiencing large amounts of traffic that isn't correctly attributed or recorded?