I came searching for a thread like this because I just now updated to 5.1 and mine doesn't work. If I type the watch face, it will light up, but it doesn't if I bump it... even really hard. I loved how sensitive it was before, but now it's not just less sensitive, it doesn't work at all unless I touch the watch face... Ugh.
I literally updated like 15 minutes ago, so hoping it comes back like it did for some of you.