@Andy > There are a few gain stages you can adjust...
1) Within the software, depending on which you run, you will possibly have the ability to adjust the channel level / gain.
2) On the controller, the channel fader and crossfader should be full-open to the channel you're wanting to have at full-volume.
3) On the controller, you have the master output level.
4) The software may also have a master level control.
5) Your speaker will also have volume control, most only have a single level control but I've seen some with two (one for an input channel, one for the master output volume).
Check that everything south of the speaker are set properly. The other issue may be that your computer isn't providing enough USB power and the output levels are limited.