Stress
As with most occupations in the sciences, if you aren't making the headway you (or your employer) had hoped, the stress monster can creep up behind you. It can be frustrating—sometimes, you may have a thousand brilliant ideas, but none of them happen to pan out.
Unfortunately, this business is all about results, and companies aren't paying you to try and fail. If you do have that cure for cancer in your back pocket, it would be an awfully nice safety net.