I keep going to catch new pets, but all of my catchers except the observer are grey, and I can't click on them to add them to an expedition. I have hired new ones since this problem started, and I have enough money to pay them, so I'm not sure what's going on. Is this some sort of bug, or am I doing something wrong?

Check your licenses. The observer is the only one that doesn't require it, which is why you can still send it out.

You can buy them in Trading > State Store

Okay, thanks