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There could be several reasons why a Spring Boot application fails to start when sub-modules are involved using Maven in IntelliJ IDEA. Some of the common reasons are:

1. Dependency conflicts: If there are conflicting dependencies between the parent and sub-modules of the application, it can cause the application to fail. To resolve this issue, you can use the Maven dependency management to manage the version of the dependencies across all the modules.

2. Configuration errors: The configuration files in the sub-modules may not be properly configured or may have errors which can prevent the application from starting. Check the configuration files and make sure that they are correct.

3. Classpath issues: If the classpath is not set up properly, it can cause issues with the application startup. Make sure that the classpath is correctly configured for all the modules.

4. Missing dependencies: If there are missing dependencies in the sub-modules, it can cause the application to fail. Check the dependencies and make sure that all the required dependencies are included.

To troubleshoot the issue, you can check the logs and error messages in the console to identify the root cause of the problem. Additionally, you can also try to build and run the application from the command line using Maven to see if the issue persists.