I started having trouble with Prosim737 not connecting to P3Dv3. After some searching I discovered FSUIPCv4 was not listed in the Add-ons menu in P3D. I checked my P3Dv3 Modules folder and FSUIPC4 and all its files seemed to be there. I reinstalled it just to be sure but when I restarted P3D there is the same problem. Then I deleted the files from the folder and ran fsuipc install as administrator. Then I started P3Dv3 as administrator but its still not listed in the Add-ons menu.