Oddly enough, I've been noticing something similar. I started using a Tickr (electrical chest strap, not optical arm strap) recently, both on my stationary and my bike. On the stationary, it's connecting to an iPad, on the bike, it's connecting to an iPhone, so the technology stack is very similar. The phone is much newer, so should in theory have less trouble. I haven't had any trouble with dropouts on the stationary, but out on the road, I've had one ride just…not pick up the HRM, and today's ride had long periods of dropouts that made up more than half the ride. I've been wondering where to lay responsibility for this.