Lack of communication i feel is some of it, letting the client know what to expect before the cleaning starts if you try to explain a permanent stain after the clean they see it as you making excuses and want you to clean it again
Ive been doing this for a long time (too long) and always thought it was common sense. You need dry air (open windows or air conditioning on) and air movement (any type of fans) to help speed dry the carpet. Well I have realized that I need to spend a couple minutes during the inspection to explain what they need to do when I leave and take my fans with me
Usually the expectation that the carpet they haven't cleaned in 10 years doesn't look "new/clean". I preach hard in the interview process about wear and tear making a high traffic area look "dirty" compaired to new unused areas under beds and such. Otherwise a few wicking spots are always easy fix.