Different performance between AudioMoth units

I recently purchased two AudioMoths for bat work here in Australia (Tasmania)- our bats have a frequency range between about 10 and 80 kHz.

One of my units detects up to six of our eight species the other only two even when they are side by side.

One of my units (two species unit) only detects calls at about one-third of the better unit.

Any ideas or advice greatly appreciated ...

Thanks

Andy