Thanks guys for your advice. The collars are a mix of new and old but
it seems to be more location that is determining whether they work.
(eg. some of the wombat collars are good no matter where while some
are bad only in the dens). They are all new except one. I can hear
them all really well when I scan them one by one but when I use them
together I have the receiver in data acqiusition mode (to store all of
the data for later download) which doesn't allow me to adjust gain or
tuning. I guess that is why I assumed it was maybe not enough gain.
The antenna outside is connected to the receiver by a RG58 cable. It
is only covered in electrical tape at the connection. It was rusty but
I have changed that all and the signal is the same. I can check the
missing data on my video cameras and see that it is always when they
are in their dens.
I think you are all probably right. Perhaps the antenna or receiver
was damaged when it was stored or moved.