I have a Symmons hot/cold shutoff in my shower. Normally, when I shut it off it goes a little beyond "12 o'clock" (more like 12:30/1:00). Anyways, I've noticed lately that it stops at exactly "12 o'clock" now and is now very difficult to turn back on. Is this because of the weather? I live in Massachusetts and we just recently had to start using the heat. Is this normal?
Most of these systems have some type of collar/sleeve that covers the internal valve assembly. I've seen them unsrew a small amount and begin to rub into the underside of the handle making it feel tight when turning off the valve. I would try taking off the handle and see if you can screw that sleeve into the wall a couple of turns.
I'm not familiar with these specific units, but if the handle isn't rubbing as @ChrisP suggests, my thought is that it might be the gaskets. That probably wouldn't cause it to stop at 12 o'clock, but I've definitely seen old gaskets cause devices to be hard to turn, and they're often a fairly easy repair.