I've got ~20 years of recruiting for startups, write resumes (and profiles, etc), and coach job seekers. 50 apps and 15 interviews isn't a bad result in most cases, so you seem relatively effective in getting interviews. Whatever you are doing to get in is working.

Sometimes the resume "oversells" the applicant. I wonder if they are expecting someone a bit more knowledgeable and skilled based on how you present yourself. This is just a guess of course, but do you feel the resume truly represents (and does not overstate) your accomplishments?

How about price? You say you've worked for 4 "brands", so I'd expect you are paid well. Maybe they all like you, but not as much as the others when comparing price and experience of others.

Your mention accent, so that is obviously at least on your mind. Doesn't sound like a huge factor if you're making it onsite 50%.

These problems can be tricky. Stay at it, and maybe ask a past interviewer for some helpful feedback. They will likely be reluctant to give it to you (legal reasons, nothing to gain but goodwill), but worth a shot.

Good luck.