Human beings are complex. You don't have to be a scientist or an academic researcher of human nature to know that is a true statement, you just have to live amongst us humans for a period of time and you understand. Well, you understand that humans are complex...not necessarily understand human behavior!
The point is, no matter how long we live or how long we have been in a particular profession, or how many experiences we have or how much training we have taken, there is always more to learn. One of the things I was always surprised with as a police supervisor was that many officers take one course on "interview and interrogation", which may be anywhere from 1 to 5 days, and they believe they have all the training they need in that area. That is simply not true. There is SO much to learn, and after 25 years I am still learning!
Police officers have mandatory training they must take each year, from firearms training to domestic violence issues, to how to use the latest computer databases. That is all well and good, but one of the things that police officers do the most, from the rookie on patrol to the seasoned homicide detective, is TALK TO PEOPLE. And not just talk to people like in a social setting or a cocktail party, they have to talk to people in a way to elicit as much relevant information from them while assessing for signs of deception and truthfulness. And they have to talk to people who are under stress, who have just been traumatized, who are under the influence of drugs or alcohol, who have elevated levels of anger and hostility, who may have memory issues, and who may or may not want to cooperate and provide them information. There are many, many more factors and complexities that make the "simple" act of communicating with and obtaining information from people so complex for the law enforcement professional.
This is one of the main reasons that it is so important to continually update your skills relating to interview and interrogation, read the latest information on detecting deception studies, associate with professional organizations on and off-line, read professional trade magazines, and attend formal training courses and academic conferences. Humans are complex...keep learning more and more about them.