Unit testing is a part of the process of developing software. In unit testing, developers verify that programs properly work as developers intend. Creating a test suite for a unit test is very time-consuming. For this reason, research is being conducted to generate a test suite for unit testing automatically, and before now, some test generation tools have been released. However, test generation tools may not be able to generate a test suite that fully covers a test target. In our research, we investigate the causes of this problem by focusing on structures of test targets to improve test generation tools. As a result, we found four patterns as the causes of this problem and we developed techniques to solve this problem.