I assume your connecting to the mixer outputs and not a stage box.
Could be…
-All of the send levels are turned down
-The mix masters are muted
-Some or all of the input channels have been un assigned from mix 3 & 4.
To check that select mix 3 and then hold down the assign button on the left side of mixer, the green SEL buttons will light up on the channels assigned to the mix, if not
keep holding the assign button and hit the SEL channel buttons to assign the channels.
– As Giga mentioned check the metering and PAFL on the mixes.
Are there various scenes saved and used on your mixer?