Each of the strips will have their own volume. That is usually a slider but on your model it's just a knob. That controls the level (a.k.a. the volume) going to the "main" mix, which is controlled with the "main" knob on the bottom right of the mixer's panel. That one controls the overall volume of the total mix.
But I guess I should ask what you are using to record with? Are you using just the webcam software? It may only be picking up a mono signal while the mains will be outputting in stereo. That would effectively cut your levels in half.
When you record, you want to turn the main up as loud as you can. You might try turning off the "2 trks to main" button as well. Try a couple of different things. Record some audio and turn the track strip levels up and down, and then leave it high and turn the mains up and down as well.
Someday we'll win this thing...