An imperfect system to be sure, regardless of its merits. People remain flawed and will let minutia affect their judgment. Overall, it's always good to stick to your guns, but at the same time, jury deliberation is the time for debate. I think you did all right, even if imperfectly. I've never served, but I sympathize, as I've been grateful to dodge that bullet twice in the last four years.
The problem with a full-time jury is that the dynamics are not random. Statistically speaking, you should get approximately the same sort of judgment from an unbiased fresh set of jurors each time. But when you get a bunch of folks who get to know each other, shape each others' opinions, and become overly familiar with the system, they produce a stagnant jury that loses the objective nature that juries are supposed to have.
As for Henry Fonda and A Dozen Grumpy Dudes, I like the film but find the ultimate conclusion faulty. In spite of several little reasons to doubt, the sum total of so much evidence still weighs too heavily in my mind to believe the kid didn't do it. It would be an amazing string of coincidences that would allow all those factors to be in place while the kid remains innocent. Too much speculation. But nevertheless, Fonda's character stuck to his guns, which if you have honest doubts, that's the thing to do.