Great at phone interviews, horrible at in-person interviews

I've been interviewing at various startups in NYC for the past 8 months and have yet to get an offer. I'm trying to figure out what exactly I'm doing wrong.

I generally get great feedback during interviews and technical screenings via the phone, but when it comes to meeting in-person I feel like I'm doing something majorly wrong.

I'm generally a very introverted person and I'm assuming my anxiety becomes apparent when I first meet people. Considering that culture fit is so important in the startup world, I'm worried that I won't be able to find a job in a startup because of it.

I've only worked in larger organizations before so culture fit wasn't as important. Does anyone have any advice or similar experiences?