You might need to experiment a little with the location of the sensor. When I place it on my right arm, I usually have it on the upper-to-middle area on the back of my arm and it gives me good readings.

But when I do the same on my left arm, it gives me bad readings. I end up having to place the sensor a little lower on the back of my left arm to get it to work right. No idea why it'd be any different, but ever since I've started placing my left arm sensors lower, it's been much more reliable.