About half of all cross-cultural missionaries return home early without completing their first term or don't return to the field after their first furlough. Here are the most common reasons why missionaries return early.
1. Sending church lowers or ends support
Churches in the West are aging and emptying. A quarter of them can no longer afford their own pastor. Many missionaries try to hang on financially until their first furlough, hoping to close the deficit then and line up enough support to return to the field, but end up staying home when it falls short. Ultimately, God keeps in the field those whom He has called, and gives them their sustenance "daily" (Matthew 6:11), which is a financial state that the modern culture calls "poverty," which is easier for single missionary men and single women missionaries than for married missionaries with children.
2. Failure to adapt cross-culturally
The missionaries sometimes, their spouses often, and their teenaged children typically lack the ability and/or the desire to learn and adapt to the local culture, language, mannerisms, climate, food, hygiene, inefficiencies, technology deficiencies, concept of time, etc.
3. Unmet expectations
Inspired by biographies of missionaries who baptized masses, as well as exaggerated reports from current missionaries, many new missionaries arrive in the field with high expectations, then become disappointed and demoralized when the expected harvest doesn't materialize, while others come for adventure and deflate after the initial euphoria.
4. Conflict with colleagues
Misunderstandings, miscommunications, cultural differences, jealousies, politics, power plays, etc. lead to clashes between missionaries, as well as between missionaries and the mission board and/or the local partners or employees.
5. Health issues
Contaminated water and food cause food poisoning, parasitic worms and amoeba, while mosquito bites lead to dengue fever and malaria. Stress and exhaustion weaken the immune system, trigger depression and other (latent) mental illnesses. In India and China, extreme air pollution causes hair loss, acne and respiratory illnesses. The lack of Western healthcare, including for children, prompts emergency or pre-emptive repatriation.
6. Exposed sexual sins
Single missionaries, both men and women, get lonely and seek companionship that leads some to fornicate, while some married missionaries commit adultery, including with fawning members of the planted church.
7. Family formation or expansion
When single missionaries marry and have children, their priority tends to shift from the Lord's harvest to raising their children, which is easier back home in safety and with family support.
8. Inadequate schools for children
International schools are expensive and found only in major cities; local schools are inadequate for preparing for US colleges; and the missionaries could feel too busy to homeschool. Some return home intending to return to the mission field after their youngest leaves home for college but many of them never return, while those who do return to the field have spent their most energetic years away from it.
9. Family obligation back home
Elderly parents lose independence and/or become sick and require care.
10. Rounded resumes
Some recent graduates, especially those with newly-minted Master of Divinity degrees, come to round out their resume and use it to land a comfortable position back home.
11. Deportation
Governments of countries hostile to the Gospel deport missionaries caught proselytizing, including to save face or to make a political statement.
12. War or terrorism
Mission boards have thresholds for pulling missionaries when war or (Islamic) terrorists approach. A rare counterexample is Free Burma Rangers.