I've been on the job hunt for a while now, and I'm wondering what I'm doing wrong. I'm getting attention from recruiters, passed every phone screen, and getting to onsites, but then consistently failing onsite. It is not because I can't write code; I've written running code in a majority of these interviews (the rest were design/nontechnical, or whiteboard interviews where the code may or may not run).

Feedback I keep getting is more of the nature "we didn't feel like you had enough experience with X." That doesn't really jibe to me because that should be obvious from my resume. It seems like if "experience" is the problem, then I shouldn't have been invited to the interview in the first place, based on the resume. A couple of times, I have gotten "It was close, but...," however, I'm not sure if I should believe that.

Assuming I can actually write code, and can do a somewhat credible job in system design interviews, what might be the problem?