It's just native speakers in general across any language. I swear I can barely understand native speakers in my second language because they all mumble or liase into fractions of words that are understood only if you've learned the specific mumbles of that accent.
"Welyuno..." "Wellyouno..." "Well youknow..." "Well, you know..."
But if you didn't know "welyuno" in English, tough luck you'll understand it in as a second language learner.
I'm replying to this comment as well to its parent.
You're both right as for colloquial speech. But we are talking about movies. Actors used to study elocution and diction techniques to polish their speech. As I said in my comment, this was apparent in older movies. Now, they don't even care about this, it seems.
"Welyuno..." "Wellyouno..." "Well youknow..." "Well, you know..."
But if you didn't know "welyuno" in English, tough luck you'll understand it in as a second language learner.