Press the "Aux" button in the Metering section and verify the output levels of the internal EFX A and B aux buses.
If the level is too low, use the Output knob to increase the master level for the effects mix. Press the Select button for each EFX bus and make sure it is assigned to the main output in the Assign section of the Fat Channel.
If you're still not getting any FX on your main bus, check out this video on internal FX routing.
See also section 4.4.2 Internal FX Bus Controls in the latest Studio Live Owners Manual:
****NOTE: This also applies to changing FX in the 16.4.2 mixer with firmware 1.50a and up.