[Jim Sims]

The first thing while obvious is to ensure you are turning it the correct way - when one is upside down and backwards it is possible to be turning it the wrong way. Second is to check for burrs or damage - most likely on the exposed male side; if it hand assembled this is probably not the issue. If I recall correctly these threads do not make a seal but instead pull two spherical sealing surfaces together; you should be using never seize compound on the threads but just a very light coating of motor oil on the spherical sealing surfaces. I think most likely the problem is correct alignment of the threads. I suspect bending tension in the hose is pulling the threads out of alignment. Try and relieve this tension while you thread the fitting on. I would not attempt to make the fittings start to thread together using a wrench, this will almost certainly result in a cross threaded and ruined set of threads. Jim