higher antennas = longer range. Thats your issue.
Also lack of error correction, poor antenna pattern to meet the low
power regulations, and a less then robust protocol, ie your probably
using some form of on/off modulation where the receiver does not have
a continious wave signal to locl on and track. Try sending a preamble
character that is 10101010 a few times, assuming your receiver is
sophisticated enough to use AGC. This sets the DC level in the
discriminator more accurately and reduces errors.

your using a radio with intentionally designed in limited range to
allow reuse of the frquency.

Steve