That makes sense, butter's been around much longer than refrigeration, I think I just needed more empty space in the jar. Once I got to camp I tried shaking it, but it was too solid to shake.

I've tried it at home a few times, and the cream goes from a whipped cream to butter and buttermilk almost instantly, but this is after about 15 minutes of constant, vigorous shaking, versus 3 hours of lazily sloshing around.

Thanks for the input, folks!