Test takings strategy tragedy
Test taking is a skill that not every student has. It should be learnable with practice, but not everyone comes to the same level. This is independent of subject. Some people just freeze up, some spend too much time on inessentials, trying hard to "get it right." Some students just panic and nothing you say will alleviate that panic. Every exam is like their first ride on a zip-line or their first rappel down a sheer cliff. 

Like anything else, all of your instruction about strategy will only reach some and everyone's response will be different. 

However, There are two things you can do. First, my own preference is to avoid stressing tests in favor of projects for grading. Tests are useful for advising, provided that you do advise the students singly when the fall short. For grading, however, projects let students do their best work in a more measured mental state. 

However, I found that open-book exams are far from optimal. The only exam I distinctly remember badly failing in College was an open-book test in Physics. The problem is that the student will study wrong before the exam and will come to depend on the book in the exam. That is why you heard the page flipping. The "facts" were in the book. I don't need them in my head. 

Prior to any exam that "counts" you want students to review and practice with the material in a way that mimics what they are likely to see on the test. They can/should provide their own review materials, or you can provide them (not as effective). The nature of the problem is that test preparation for an open book test is insufficiently active. They don't do much to reinforce their own knowledge beyond, perhaps, knowing a bit about where they can find various things in the book. 

Moreover, your test isn't likely to just ask them to transcribe the book's information, but to do something with knowledge learned. Open book tests actually discourage the actual learning part. 

An alternative to open-book is to permit the student to bring one standard sized sheet of paper to the exam on which they may write anything they like prior to the exam. The paper should be signed by the student and handed in with the exam (largely to prevent student B from using a copy of student A's sheet). This forces them to be at least a bit active in their prep. 

A second, possibly valid, use of open-book testing is the following. Assuming that the test period is about an hour, announce beforehand that for a five minute period starting at the 30 minute mark, students may consult their books and/or notes. The open-book segment needs to be short and it needs to be after students have had a chance to consider each question. 

tl;dr: Consign open-book tests to the dust-bin of history.