Synchro Copurs are overly complicated and delicate. Flushing will spread dirt and lube to new paces. It needs to be stripped down,cleaned, relubed, and reassembled, as has been said.
The only Synchro Compur I have brought back to life was on a Rolleicord Va. I added three very small drops of oil, one to each main pivot in the slow speed mechanism. If it hadn't worked, I would have been looking at a complete strip down in my experience.