I've drawn all 3 nipples the same way, and in the assembly, all but one nipple has been able to be mated the same way.

I click the OD of the nipple and mateit with the ID of the hole it's going into. Then I mate the ID of the main pipe with the cut face of the nipple and everything works fine.

But for the 3/8 nipple I CANNOT get it to mate the same way as the other 3, it tells me the mate will be undefined.