Why is findclass not finding classes?

Question

Java

There is a desktop application written in Java which consists of: main.jar, app.jar, lib1.jar, lib2.jar...
The application starts from main.jar and loads app.jar (the application itself and its functionality) and additional libraries. The app.jar is loaded like this

Class<?> clasz = classLoader.loadClass("some.class.to.load.from.app.jar");

jsize count;
  if ( JNI_GetCreatedJavaVMs( &mc->m_jvm, 1, &count ) != JNI_OK || count == 0 ) {
    return;
  }

  jint res = mc->m_jvm->GetEnv( ( void ** ) &mc->m_jenv, JNI_VERSION_1_6 );
  if ( res == JNI_EDETACHED ) {
    res = mc->m_jvm->AttachCurrentThread( ( void ** ) &mc->m_jenv, nullptr );
  }

  if ( res != JNI_OK ) {
    return;
  }

jclass classs = mc->m_jenv->FindClass("some/class/from/main/jar");

This way I can access classes, methods etc, but only within main.jar

jclass classs = mc->m_jenv->FindClass("some/class/from/app/jar");

In this case, findclass returns null.
The question is, how to access classes from app.jar?