Garmin's heart rate monitor

So I'm starting to hate Garmin's heart rate monitors. Today I started a run and my heart rate hit the mark that I was feeling but at about 23min my heart rate started to drop but I was still maintaining my pace. I finally started to notice my heart rate was dropping at 130 and I was sprinting at that miment but it still only went lower and it was at the same spot on my body. I even removed it try to dry it and shift it around on my body but no difference. With this all I missed my requirements to get my point on discovery vitality and this isn't the first time or my first heart rate monitor with them. This is utterly useless I'm doing everything what they told me to do and it's not bend and they are the best I don't think so anymore.