If you are working with a 3rd-party virtual instrument or FX plug-in, and you have zero latency monitoring turned on with the green "Z" in the Main Output in the Console, Studio One will disable any plug-ins that do not support zero latency. If the plug-in is supported in zero latency monitoring mode, then the power button next to the instrument will be highlighted in green for the track that you are working with. To turn off zero latency monitoring for virtual instruments, you can go into Studio One>Options>Audio Setup>Processing and de-select the box. You can also de-select the green "Z" on the main output of the track. If you continue to have the issue, please log into your my.presonus.com account and submit a support ticket. PreSonus would be happy to assist.