This doesn't happen to me anymore, but when I was younger, people (mostly my classmates) used to think I couldn't talk at all. I had selective mutism, which means I would only talk to certain people (in my case, those "certain people" were family members). This made me so mad when I was younger because I knew I could talk, but I couldn't bring myself to talk to my classmates. It took me years to ever speak to my best friend.
I guess another thing is that people might find me boring? I'm not boring, trust me, I have tons of personality once you get to know me. I'm reserved when I'm not around friends and don't talk to anyone else, so I guess that explains why people might think I'm boring.