I think it would be helpful if you could be more specific about your environment. I take it you have 2 physically separate installs; one allows process enablement, the other does not?
If you’re certain your MWS is configured correctly to talk to the Integration Server, the next level of debug I’d suggest is to set the Logging Level for the Process Engine (subsystem 101) to DEBUG, try to enable the process again, and manually inspect the IS log for indications that the PE received the request and invoked the appropriate service (setModelEnabled). If the log is eerily silent, I think you can conclude that your MWS is not actually talking to the IS in question.
To rule out the stack below MWS, you can manually execute the setModelEnabled service in your Integration Service. Specify the Process ID (and version) at the appropriate input fields and let it execute. If this works, you can rule out any problem with IS or the Process Engine and suspect the link between Monitor (MWS) and the IS.
Please post again with an update to your env and results of this advice.