If your device shows only heart rate when it is turned on it is in a "HR only" mode. In this mode it can be used with 3rd party apps. Please see more at:
To exit HR only mode and to use your device with PulseOn application, you will need to pair it again. To set PulseOn device in pairing mode, please keep both buttons pressed down for 7 seconds.
In case of pairing issues, see also: http://www.support.pulseon.com/support/solutions/articles/5000502790-pairing-problem-my-application-does-not-synchronize-with-the-device-