What to do about a non-functioning US to CW trade resource, indicated by no convoy points being used:
It took me awhile to figure out how to fix this. Set the Canadian resources being shipped to the CW to "Idle". The point is to set clear enough unused CPs across the Atlantic so that the US will be guaranteed room to ship its goods. You might have to turn off some CW points going through the Caribbean, too. Recompute. The US trade resources should now be using convoy points, which means that they are functioning. At this point, you can start turning on the resources you had turned off.
P.S. Other production quirks, with fixes:
You often have to manually change routes so that resources/build points going across the Atlantic go through the Faeroes Gap, not the Bay of Biscay.
Set the Fairbanks oil so that is not being traded to the UK. Pick some other resource, send that, and send the Fairbanks oil to wherever the one you just traded was going.
If the US is trading build points to an African based Free France, MWiF will insist on sending build points from Washington, usually completely snarling your convoy routes. This is easily changed to sending them from Sao Paulo, which is where you wanted them to come from to begin with. Unfortunately, MWiF has a nasty habit of silently changing them back to Washington at some point, and when you find this out in Final Production Planning, you are not allowed to change them. There is an easy work around. Save the game in the Allied Break down unit phase. If MWiF does this, or any similar nonsense, you can go back to that save, change the production there, and the changes will stick this time. (At least, for me, so far, they have always stuck. No promises about tomorrow. This is a computer we are talking about. )
Other build point paths have the same problem (CW to Russia or China when they want to use Indian build points, for example.) The same fix works.
I thought I knew how to play this game....