stars are not gained from halter shows, they become visible after you register them to their breed registry (for purebreeds) or game owned club (for recognized crossbreeds such as Walkaloosas). you have to have your horse registered in order to halter show.

Non recognized crossbreeds/campaigns cannot be registered and therefore halter show 

You can technically register Grades into Made the Grade club but you cannot halter them at all. Only what everyone above has said can be haltered only.

As well, just double checking in. Are their working paths selected on Halters only?

@everyone: Thank you! That's super helpful.

@TheDustyAngel: They aren't; can horses not compete with a discipline and halter show at the same time?

They can the boxes just need to be checked

Personally, I don't do both Halters/Disciplines at the same time, it's either one or the other but you can play the game however you want and as Maki said, just make sure their boxes are checked and Career/Explore is not checked so you don't inadvertently use the horses for Careers/Explores as I've done that a few times myself.

I only asked about Halters only being checked as I'm used to newer players forgetting to double check the horses' working path and a path isn't checked, so wanted to double check that as a possbility as to why you couldn't Halter show some horses for.