low volume with usb mixer

Hi, i'm running Garageband 10.3.5 on my macbook pro to record mixes on my technics turntables, through a Behringer usb mixer.

When I record, the volume level slider is greyed out, so am I right in assuming the volume is set on the mixer ? If so, i have the gain turned up pretty high on the channels and the channel slider up high too, but the volume is quite low after saving to my music.

Does anyone know what I'm doing wrong here ?

I have recorded one mix that came out with perfect volume, not sure what I did but now I know it is possible.

Any help would be greatly appreciated.