Range Problem (S1 Pro Xbee)

Recently I was experimenting a wireless sensor network where I used a S1 pro xbee, one of experiments was a weather condition simulator and test its effect on the packet loss. I did the experiment and I made sure that the xbee is protected with a plastic box in the weather condition box simulator. However, after I did the test I started facing a range problem, the range became short.Therefore , I bought new xbee S1 pro but after a while of using it I started facing the same range problem. Does anyone know what causes this range problem?