I'm a newbie, done a couple of courses and know how you feel about carpets not looking as clean as you'd thought they could be.
I've found that a lot often it can be wear to the pile. I make a point of highlighting this to the customer as part of the survey and get them to look and note the difference in the pile etc. It's about managing expectations and not over promising I think, and, if it does come up well you're the hero in the client's eyes.
I agree with the dwell time and agitation comments also. I found that on the first few jobs I did I was being too quick with the wand, once I slowed down on the initial pass I found the results were better.