Is the screw too long? I have that problem mounting the RDF on the FLT98, and need to track down a screw about 2mm shorter.
Your other choice would be using a thread locking compound on it. To be on the safe side while applying it, I would remove the focuser assembly from the scope by backing off the three nylon grub screws and then pulling backwards on the focuser. You would likely want to use the non-permanent type