Sport Watches

Hi i’ve got a Garmin Forerunner 50 watch with the strap that goes below on your chest to sense your hate rate. I get to that stage where the watch screen comes up with Heart rate FOUND…then it flashes up 54…then just nothing…the screen shows the two little horizontal lines where the digits should be but it doesnt show my actual heart rate. Does it just flash up every now and then or should there be a constant figure on the watch screen?? I just tested it standing still so the strap didnt move position…nothing again…then when I went for a cycle & still nothing just the two horizontal digit lines. Can anyone help?