I've gone through 3 HRM-Run devices. The typical scenario for me is that it stops recording certain aspects of my run, such as running dynamics or heart rate, but usually records something. The first 2 were replaced under warranty by Garmin.
If not done already, I would start by replacing the CR2032 battery. Make sure to leave the battery out for 5 min, before you insert a new one. Be careful not to over-tighten the screws, as the plastic screw holes can crack. Thereby making the HRM more prone to moisture issues.
This didn't help in my case, so I contacted support and they shipped out a replacement at no cost.
Ok, thanks. I'll contact support then. I live outside the US, though.
For me, it records the run dynamics and even the ConnectIQ power widgets work, but no heart rate. After the issue first appeared, I was able to some how get it back working, by playing with different exercise modes and re-connecting the strap. But now nothing works.
There was some recent discussion on this in the Accessories & Sensors forum.