In the movie, Star Trek: First Contact, the Borg can "adapt" to withstand shots from the weapons that Picard and his crew carry. The Borg also ignore Picard and his crew until they are deemed a threat, and need to be assimilated. If this is the case, then why do the Borg continue to ignore Picard and his crew in different events, and not "adapt" to recognize them?
Realistically, the Borg are faced with a slightly different adversary every time they come up against Picard and his crew. It is important to remember that not all of Picard's crew were constant. Compare the crew of the Enterprise-D and that of the Enterprise -E. You will observe there are different tactical officers, which is mighty important :when it's Worf we know Picard will reject his suggestions! On a more serious note, considering that the tactical officer will give suggestions to the Captain for a course of action in a tactical situation, this could seriously change the dynamics of how Picard will respond.
Also, it's important to understand that Picard is an individual, a concept the Borg have great difficulty understanding. Hence, their ability to predict Picard's course of action could be limited in this regard. As pointed out in Mario's answer to the aforementioned question, First Contact depicts Picard as being unusually aggressive!
Furthermore, they would probably appreciate that individuals can change and therefore acknowledge that, despite having assimilated the experiences of Picard during 2366, the events of First Contact are set in 2373 - 7 years afterwards. A lot can change in 7 years and Picard, as well as the Federation no doubt, has had dealings with the Borg during those 7 years.
Also, considering that the Federation had made moves to address the Borg threat, at least in terms of developing the Defiant class, it seems quite reasonable to expect a change in tactics when dealing with the Borg have been developed also.