From my tutoring experiences, the most daunting barrier that I have encountered is test taking anxiety. This is usually apparent when students who take tests in the comfort of their home score extremely well. However, once they wander into the unfamiliar test center territories, reactionary reflexes takes over, and the test becomes a tale of struggle and survival! Or the test becomes extremely difficult to the point of being overwhelming.
In fact, one of the ways I help students overcome this test taking anxiety is to familiarize them to the conditions that they will experience on test day, including working with mimicking exam conditions, and having them take a complete test in one block of time with breaks reflecting the actual test as much as possible. Also working on approaches to questions commonly seen helps to increase their resourcefulness when a challenge is encountered.
Usually, the first practice examination is a shock. The second one does become more comfortable. The difference in scores between the two many times is a large one, at times up to several hundred points, for example, on the SATs. So while I do enjoy teaching in a controlled environment as this is ideal, I stress building test taking tolerance in the field to prepare for a less than ideal setting.