This is insanely frustrating to most people, and it takes a certain kind of person to persevere. Success is not just a function of work or intelligence, but to a great extent of chance. This is hard for most people to accept. It is frustrating to work twice as hard as a classmate, and yet accomplish only half as much. Unfortunately, this scenario is common.
Science is also very political.
Scientists, I am sad to say, are often petty and sometimes unscrupulous. Multiple labs often work on the same projects and compete with each other. It is agonizing to be scooped by another lab, or to fear that you will be scooped. You might believe that you have lots of lab experience already and that you know what you are getting into. This may be true, but graduate school will not merely be a continuation of your college lab experience.