The Y-Comm seems to be quite unreliable at updating itself, so I have found a technique that has led me to get many gigantamax Pokémon, and can help you! I see a lot of people complaining they aren't seeing the invites when they run out of raids.

You need to go into your Y-Comm, then select "Swap Cards" then set a link code of a random number no one would ever guess, then let it search for someone to trade cards with. It shouldn't find anyone because of your link code, but this forces the Y-Comm to constantly update itself at a much faster rate. You can also then keep closing and opening your Y-Comm to get a new set of invites.

I've got SO MANY Gigantamax forms this way.

You're welcome!