There is no bell curve in language acquisition. If a person has normal neurological function, normal hearing and speech, there is no bell curve — everyone is a native speaker of their first language. Why should we expect anything different from a second language? Well, because we’re preventing that acquisition from happening in many cases. It’s really sad.
This is a great point. The fact that everyone speaks their first language natively is solid proof that they have the ability to learn and speak a language like a native. There is no bell curve of low, average and high fluency.
This is not to deny that people will have different levels of knowledge. Obviously native speakers of any language will vary greatly in their amount of vocabulary. But they are all native, fluent speakers.
So, if someone is struggling to learn a second language, the problem is with the method, not with the person’s lack of ability.
(This is why GPA is so effective, I think — it mimics, in a very smart and careful way, natural language learning.)