Sounds like supply issues. Make sure:
- You have a level 10 port in Canada (and you're exporting from a level 10 port too...)
- You have a ton of convoys
- Your convoys aren't getting sunk (close sea zones if needed, and use your ships to convoy escorts those that are open, making sure they have sonar and depth charges against US subs)
- Use logistics companies on all your troops, maxed out
- Prioritize quality over quantity of troops so you don't exceed the province supply
- Watch supply status like a hawk - you might already know this, but in the cluster of buttons at the bottom right, it's the one on the top right, with the little star. Anything other than green is bad - it'll also show you how your supply is working (or not working).
As for attacking the US: the earlier the better. They have as almost as much manpower as Russia, vastly more resources, more industry, 6 research slots and massive research bonuses, so they will snowball so ideally you want to catch them when they're weak, early on (as soon as you've dealt with the Allies, like 1940). The only (very tedious) alternative is to wait a looong time as eventually they'll run out of building slots and so you'll dwarf them, but that'll be insanely tedious.
PS: It sounds like you've already done this, but make sure you invade Canada from Greenland, just one victory point is all you need, before you defeat the UK, so they'll be up for grabs in the peace deal.