Cover has absolutely nothing to do with it, I never use cover and it is nowhere to be found on any hot bar. However here is a video of the ability failing. I have also noticed this on a few other abilities (like corrosive dart as well) and I believe that it is latency related which really sucks because my ping is only 100. It seems that if you spam the button it works better but pressing it once sometimes with server lag it doesn't see it correctly however the ability still casts but doesn't actually heal. Video - http://www.youtube.com/watch?v=3fc79BNC-zI