The GL3300 XLR outputs are all balanced.
The balanced option applies to the Aux jack outputs.
If the XLRs are giving low signal, the problem could be: -
Disconnect all cables from the group inserts. Does this solve the problem?
If a single XLR is giving approx 50% output the then maybe the output has lost either + or - signal (short circuit/failed IC/bad solder joint).
If all group XLR are giving low signal:
Make sure the input channel pan control is fully right for groups 2, 4, 6, 8 and fully left for groups 1, 3, 5, 7.
Route the input to the master Left at the same level as the group.
Plug headphones directly into the master Left insert jack.
You will hear signal on the left earpiece.
Now plug the headphones into the group insert jack.
If the signal is the same level as the master Left, the problem is likely to be with the external equipment connected to the group, or incorrect wiring.