Not been able to gain or show the difference in Altitude

Im using mission planner to fly my quadcopter but when testing the settings to see if everything is working it shows the compass direction and degrees but it wont show the difference in altitude when im holding it normally it says 0meters and when I hold it above my head which is over 2meters it still says 0 I stand on a ladder about 6meters high and it still says 0. Do you know why it may be doing this?

What model APM are you using. Do you have the barometer covered with light foam?
Have you used the CLI in the MP to test the barometer?

Im using an AMP 2.5

What part is the barometer?