Working on a site at the moment and 99% of traffic is from the PPC. Its amazing how many return visitors there are that come direct or through brands on the search engines and GOOGLE ADWORDS does not track... Anyone experience the same?
Adwords cookie only tracks for 30 days. Enable the transactions option in reporting to see conversions from repeat visits. Install Analytics to get better reporting for conversions from all sources and repeat visits.
We only get a small percentage of untracked from PPC, but I think it depends on your niche. For example, someone finds your site via a search at work, then goes back later from their home computer to finish the purchase. You can also try adding your own tracking cookies to make sure things are working, and/or add google analytics to the mix to see if anything strange is happening.