how to set the right range for data transmission?

hi,
i have some problem regarding the range of data transmission. from the datasheet, it says that the outdoor RF-line-of-sight for xbee pro znet is 1mile(1.6km)…is there any configuration that need to be set? i have tried it outdoor but can only received data at 100m and if exceed that range the cannot received data…what did i do wrong? can anybody help me…