When you are unable to keep a consistently green "pulse" it is more often than not, a connectivity issue.
For all users, make sure the HR strap is wet and fitted securely around the base of your sternum.
If you are using a wireless receiver, try removing the case from your device, ensuring the receiver is plugged all the way in.
If using the wireless receiver, make sure the device is held relatively close to the HR strap (transmitter).
If using a wireless receiver and you have just updated your device software to iOS 7, make sure to go into the settings --> privacy --> microphone --> enable BioForce
For those users with Bluetooth smart HR straps, make sure to turn Bluetooth on in the device settings, as well as in the app itself. To do this, open the app, and click "edit" in the lower right portion of the screen, and make sure "use Bluetooth smart strap" is turned on.
If all else fails, try replacing the battery in the HR strap transmitter.