In that case the issue is the device you are using. There are some devices that do not supply the user with the altitude variable and this is why you cannot set your POIs to different altitudes.
Did you already try to give a value to your altitude variable (as described here) and it is not working?
Yes I did but all markers were still on the same level/height.
I am trying to do multiple POIs with GeoLocation but the altitude is not working. All markers are on the same level. Can you help me on this?