Inspect the threads on the brass, there is a posability that there is thread damage which may be cutting into the cpvc fitting as you screw it together. A tiny file will smooth the ding if there is one and the threads are outside (male) ones. If such is the case with inside threads then you need to replace the brass fitting or run a pipe thread tap into it. If you need to run in a tap get a 'bottoming' style tap, the older guy at the hardware will know what you are saying, the young ones won't.