What frequency range are you using? What distance to the NCS?
The CHA-250 is not particularly efficient (sometimes referred to as a "dummy load on
a stick") and a vertical is not ideal for NVIS working (the typical propagation mode for
contacts out to 200 miles on HF. Because there can be a fair amount of radiation from
the coax, you can try running the coax horizontally across the roof to optimize high angle
radiation.
Another possibility is that the conditions have changed in the last couple of months and
the ionosphere no longer supports the required path on your net frequency. (Make sure
they didn't make a seasonal frequency shift.) For such short-range nets the frequency
needs to be below the "critical frequency" at the time but not so low that D-layer
absorption is excessive.
A quick test would be to put up a temporary dipole or loop, even if near the ground,
and see if you can hear them: that will tell you whether the problem is propagation
or your antenna.
You can also check the propagation forecasts for the path using online tools from the
Australian Ionospheric Prediction Service (
www.ips.gov.au/HF_Systems/7/1) or
VOACAP Online (www VOACAP.com/prediction.html).