Data Transmission of multi-sensors through Xbee

Hi,

I am using Xbee series 1 which mounted on the Xbee shield to transmit data from my sensors (accelerometers, FSR, flex sensors) to pc.
It transmitted just fine in a short range (3 meter), but when it goes up to 4 meter and above, some of the data from the sensors cannot be transmitted.
I realized that Xbee transmission range can go up to 30 meter. So where is my problem?
Is it because transmission time for multi-sensors is too short?

Thank you