Why are my black lines thinner than others? What’s the secret?

Hi everyone! I’ve been tufting for a bit now, and I’ve run into an issue when it comes to creating fine black lines for outlining designs. It feels like using just 2-3 threads for a line isn’t enough, as the result is too thin.

However, when I watch tutorials or videos on YouTube, I see people using just two threads for outlining, but their lines come out much thicker and more defined. I’m wondering what I might be missing. Are they possibly using thicker yarn? I usually work with yarn that’s 3.5-4mm thick. Should I be using a 5mm yarn instead?

Would love to hear any advice or tips! Thanks in advance!