range issues in XBEE S2C module

I am using XBEE S2C module as a coordinator,router and end device. My products are in ready to dispatch state, but I am facing a problem of range.
Two end devices are kept at same positions besides each other and coordinator at a fix location. Both the End devices being at same position they show different RF strength. There is also a variation in the RF strengths of individual devices(approx 20%). Now among the two end devices one is showing 20% range and other 4%.
How such things can result in so much of difference.
What are things i may have missed out to be done for XBEE settings. Or some things i may have not noticed which can be the reason for this.
Kindly suggest.
Thank You

What is the exact part number of the two XBee modules you are working with?

What is the firmware version installed in each?


firmware version : 4059

antenna used is : P/N:A24-HASM-450 (2.4GHz, 3 dbi)
distance between coordinator and Transmitter is hardly 3 mtr. We have made 16 networks(16 coordinator) each having 5 End devices configured.
For S2C the signal strength should be measured between -26dbm to -88dbm.
We are using Remote DB command for measuring strength
We have observed signal strength for 2 hrs with fix position and is receiving signal strength between -63dbm to -42dbm, which shows a fluctuation of 21dbm.
All 16 coordinators are giving same result.
The same problem is observed on field where some end devices are communicating with good strength and same one’s are loosing communication for 2-3 hrs in between anytime.

Sounds like you are having interference issue on the channels selected. Or simply have too many networks on the same channel and in range of each other.