@hurdle
The fact that you could hear the FX when set to pre-fade but not when set to post-fade suggests either your main channel or FX return fader (to LR) is turned down or is being affected by a DCA, so these would be the first things to check.
It depends on your workflow, but a common setup is to have all FX sends set post-fade and all mix sends for monitoring are pre-fade (including FX returns). This means delay based FX (reverb/delay) will follow the main mix (dry) levels on the sends side and the returns side level is used to set the independent balance for each mix.
Cheers,
Keith.