Assuming that the failure of an expatriate is not related to immigration matters, such as failure to secure the proper visa, but rather a failure to successfully assimilate into his or her new country, then the cause is almost always cultural. Whether the issue is American expatriates living abroad, or foreign nationals sent to live in the United States by their employers, the inability or unwillingness to abide by the cultural traditions and legal requirements of the host country is the major cause of failure. Occasionally, the problem is entirely a failure to adequately learn the language of the host country -- a more difficult challenge than one might assume given the specialized vocabularies often involved in multinational professions -- but companies are unlikely to dispatch abroad employees who have not already demonstrated adequate knowledge of the required language.
Far more often, foreign nationals residing in the United States at the behest of their employers fail to adequately assimilate. They know the language, but they reject the strictures of their host nation, and resent the confinement that the expatriate community sometimes endures. American expatriates living in countries like Saudi Arabia and China usually assimilate quite well, but some don’t adapt to the cultural, religious (in the case of Saudi Arabia) and legal parameters of those countries. Similarly, foreign nationals living in the United States have encountered problems with American customs and laws restricting public behavior. This is the “single point of failure” for expatriates.