Five problems in five interviews sounds low. Obviously this is pure speculation, but perhaps the concern was that you didn't solve them fast enough. Interviewers are supposed to ask more than one technical question; the candidates I am enthusiastic about hiring tend to solve 2-3. That said, some problems are multi-facted enough that an interviewer can run with the same basic theme for the whole interview, I just think it would be unusual for that to be the case in all five interviews.
I actually had a couple of interviews with more than one questions, but not others. I agree this could have been one of the reasons, but again an indication of this in the feedback could have helped.
