You need to tell us how you have this wired. WHat channels of the TIU? If loop 1 which works is powered off Fix 1, and Left handle of Z and loop 2 is on FIx2 and right handle of Z, the first thing is a simple swap of Z input power. Move the right handle output to Fix 1 input. IF the loop still works you know both outputs of the Z-4000 are good. SO it must be TIU or wiring to loop.
Remember VAR channels must be turned up the control track voltage not the transformer. IF both fixed channels being used I suspect a fuse on the non working channel. G
