I am playing in a party with friends (not split screen) and yet I'm not unlocking anything. I don't have everything because my inventory has like 6 things in it. Plus my friends have antennas and toppers that I do not. What do I do?
You are likely hitting up against the 10 hours/week drop limit. From the patch notes:
There is a maximum of ten hours of playtime per week that can earn items, so don’t bother trying to idle your way to a full inventory!