I just put on replacement tires and tubes. I had to work to get that nut to catch also, but finally did. If I recall, you have to try to center the stem as much as possible to get the nut to catch.
John Bender
Has anyone had issues putting the washer and nut on the tub valve stem?
I have one wheel together, but the other one doesn't have enough thread for the nut to bite.
Look at the bottom of page 16 in the Matco technical service bulletin that comes with the wheels. It says that with some tube brands (I have been told by Matco folks that Michelin in one of them) you need to replace the special washer with AN960-516s. All I can say is that it worked in my case.