Clinical studies, like those by Barbara Mason at Scripps Institute, have documented a marijuana withdrawal syndrome among a minority of users. Are we prepared to say that marijuana is addictive? Why didn’t we identify this syndrome years ago?

Nora Volkow: Absolutely, there is no doubt that some users can become addicted to marijuana. In fact, well over half of the close to 7 million Americans classified with dependence or abuse of an illicit drug are dependent on or abuse marijuana. It is important to clarify that while withdrawal is one of the criteria used to diagnose an addiction (which also includes compulsive use in spite of known adverse consequences), it is possible for an individual to suffer withdrawal symptoms without he or she being addicted to an abused substance.

Now, to answer your specific question, the reason for the relatively late realization that people who abuse marijuana can develop a cannabis withdrawal syndrome (CWS) if they try to quit is probably the result of at least two factors. First is the fact (which you hint at already) that a clinically relevant cannabis withdrawal syndrome may only be expected in a subgroup of cannabis-dependent patients. This may be partially explained by marijuana’s uptake into and slow release from fat cells, which can occur over days or weeks after last use. Thus, cessation of marijuana use may not be so abrupt, and could thereby diminish signs of withdrawal. The second factor relates to the small to negligible associations between recalled and prospectively assessed withdrawal symptoms, which may have precluded many previous, recall-based studies from detecting or properly characterizing CWS. It is also worth pointing out that other addictions (e.g., cocaine) were also not initially thought of as capable of triggering withdrawal symptoms.”
Source: http://addiction-dirkh.blogspot.com/2009/12/q-with-nora-volkow.html?