Some combinations of nipple and cap do not match up. If thats the case here and it sure sounds like it; then what I do is adjust the fit by carefully [don't damage the threads] put the thread end of the nipple in drill press or drill chuck and lay a file flat against the part of the nipple that the inside of the caps side goes against and carefully turn it down just enough that the caps will fit right, do not take off more than you have to. Lee