I was starting to panic, as I am running the Liverpool half-marathon tomorrow and the chest strap monitor that came with my Garmin Forerunner 305 suddenly stopped working yesterday. Previously there had never been any problem with the Ant+ device - the HRM1G - so I was a bit concerned.
After replacing the battery twice, I was no further forward. I even tried resetting the Forerunner unit itself.
A couple of Google searches later I came across this thread suggesting reversing the battery. I tried it, and the HRM1G registered straight away with my Forerunner unit. How very relieved I am.
Although the Garmin forum thread suggests that the polarity has been reversed, in effect the upturned battery is just short circuiting the two contacts due to the shape of the battery.
A useful hint to know.