Your Yukkuri's crying alot, nothing will stop it. Could it be that if it had sisters, and you squashed one of them? (http://yukkuri.wikia.com/wiki/Squished_Yukkuris.)
If so, tell your yukkuri, "I'm very sorry I squash one of your sisters, that was really selfish of me." Of course, if Yukkuri insanity is what you want, don't bother, selfish rude person.
Isolating self, refusing to play.
If you're a kind person and you don't hurt Yukkuris, then it must be something else. One of those 'something else'... Is isolation. Noticed your yukkuri stays in corners of the house and says, "Sowwy, Weimu doesn't want to pway with mister ball today." Or something like that? Take your Yukkuri, and ask if there's anything you can do. Unless you adopted the yukkuris, make sure to take it to it's mommy or daddy. In other cases, talk to the Yukkuri shop employee; Yukkuris (ESPECIALLY koyukkuris) are playful creatures, therefore it's not normal if it doesn't want to play.
Sometimes, when you talk to your Yukkuri, it says, "Yu... I can see mishter buttewfwy on the ceiwing", or a random thing that's not true. But if you have a Koishi, you shouldn't be reading this at all, they're already pretty crazy. But if any Yukkuri but a Koishi, go to the Yukkuri shop and talk to the shop employee. If they have a vet, talk to them.
In all cases...Edit
If all these are true, here's what you need to do out of these:
Go to a Yukkuri vet.
Your Yukkuri's not in a good state: You need to get it to a Yukkuri vet. It might be scared at first, but when the operation's done, have a moderate visit every week.
Give back the yukkuri.
Did you Read 'Reimu's malformed sister'? The crying and all? That's what might happen to the other Yukkuris when they'll see one insane. They'll say, "Mwommie, why is sister wike that?". Therefore, you must give away the yukkuri to its shop; The vet will take care of it.