Double check the Zoopedia for the numbers that Fossa will accept socially. I think they will only be happy alone or as a mated pair (plus their juvenile offspring) too many adults will make them unhappy regardless of how big the enclosure is.

sometimes the game will struggle to process chaqnges in the social dynamic. for example, I've seen the issue of a female fossa that was a daughter of a mated pair that has grown up being considered still an outsider after I removed her parents and introduced a new male. To resolve this I sent her to the trade centre and put her through quarantine as if she was a new arrival and then she integrated with the new male and continued happily.

Thank you! Yeah, I only put one of each in, hence why the issue was so weird. I'll try and see if putting them through quarantine helps. :)

Sometimes nothing works and you just have to give them time and then they will be fine. Happens to me sometimes. Frustrating, but if the tricks don't work, you're stuck with just waiting.

EDIT: Happens to me with a wide range of animals I should say, not the fossa in particular.

Putting them through quarantine helped!

You know, I usually put them in the trade center. I'll have to try the quarantine myself next time. Thanks for the update!