Nipples

Some cars, I don't pay much attention, one hose is 1/2 and the other is 5/8. Is it possible the nipple you have is simply the wrong size? Also if that is new "china" hose, that could be suspect for size. I recently bought what was supposed to be 5/16 hose and it was more like 3/8. I've NEVER had problems clamping 5/16 hose to 5/16 tube (this on my tractor) and I could NOT get that junk to seal.