Is the student having difficulty because they are an English Language Learner or because they have an a learning disability?

All students develop and learn at different paces and have different learning styles.  ELLs come from different educational cultures and are trying to adjust to a new social culture an learning environment, and as teachers we try to aid them in the adjustment. Although research shows it can take 7-10 years for an ELL student to become proficient in English, some students seem to progress slower then others.  Students that have been in an ELL program from kindergarten through fifth grade it seems should be at the high end of the Developing Level or Expanding Level, but when accessed are still in the Beginning Level or low Developing Level.  I have seen students referred to ELL because they do speak another language and are struggling in reading and content areas. The student may speak another language but their dominant language may actually be English.  Is this students struggle do to language or a learning disability?  Teachers need to carefully access their students so we can determine what is holding them back.