It sounds broken. Erratic HR monitoring has been reported in this forum many times. It seems nobody cares to fix it. I rarely earn zone minutes when I am active, but I do get some now and then when I am just sitting around. Everything that depends on accurate HR is suspect.
You might be able to get a replacement if you call customer service. They did send me one when my first watch was way off base. Now the replacement is erratic as well. I think they have some major bugs that need fixing.
I hope someone else comes in with a better answer for you, because I'd like mine to work more reliably too.
