I have a client with an online shop whose backend system show some 50000 transactions for the last year. Google Analytics show 48000 Transactions (all numbers are by way of example only) which is close enough.
According to the Google Adwords reports some 45% of conversions are generated by Adwords. According to Google Analytics only 30% of those conversions are CPC conversions. Since GA reports a (broadly) correct number it looks like a lot of conversions are attributed to the wrong marketing channel. The numbers of clicks in the Adwords report and the number of sessions initiated by Adwords roughly match up (with a difference of about 5%).
However performance does not suspicious in the other channels. If anything is wrongly attributed it must have ended up either in organic traffic or direct traffic (the other channels have not enough volume to contain the difference). There is no sudden shift of conversions rates between channels, so whatever is wrong has been wrong from the start.
Now, this sometimes happens when campaign parameters (or the gclid parameter from Adwords auto-tagging) are lost (e.g. in redirects). When testing I don't see any evidence of that happening, though.
I'm a bit at a loss now. How would I proceed to debug the problem ? Any ideas welcome.