Thanks guys. You have given me many things to try. I think I'll start with ordering the small strap. Even though safety pinning mine hasn't seemed to help, maybe the smaller strap will somehow help. I have noticed, actually, that my strap loosens when I wear it (but still didn't work right at it's tightest).

I will be careful to clean the sensors.

I will try to lower it and see if that helps. (Already tried putting it "under" the actual bra strap. But typically it goes just below the bra strap.)

I'll look for the Polar sports bra to attach it to.

And if none of that works, I'll resort to trying the older, less comfy strap. Hmmmm. Dh might even have one I could try right now.

I'll keep you posted on my progress.