Always test run it in conventional. If it won't do anything, no sense looking further than the engine.
If after trying an add, you notice an address of a certain engine keeps popping up, try pressing start and see if it starts. The remote may think the engine is already added. So it won't change the new engines board address.
If it's a lash-up that comes up, you should edit that address and then try to add the new engine again.
I lost track if the newest DCS 6.1 has (fixed or) changed this behavior. When all engines aren't getting power, the remote doesn't know that the address used is not for the new engine if it's the same. It helps to know what the address is on the new engine and clear that address in the remote.