How do guarantors wriggle out?

Guarantor

I have a wealthy guarantor of a housing benefit daughter who is driving me mad.

Usually when I present the end of tenancy bill to guarantors they put their head in the sand for a month or 2 and then pay up - sometimes after court action - sometimes before. This guy is different. He is fairly intelligent and seems highly motivated to keep asking me for more and more information in a way I am not used to. eg knowing that I needed a possession order he has asked for a court receipt for the £355.
After providing lots of information and rent statements and inspection reports etc I have told him that his role is not to manage me but to pay what the tenant does n't.
So what I am really wondering is where are the vulnerable points to guarantors attempts to evade payment?
Thus far I have generally been successful apart from one failed county court hearing where the judge was reluctant to let me charge a guarantor rent for a tenant who moved back in after leaving. He said there was a break in the tenancy and so the guarantor was off the hook.
Other than that I have always got my money.
I would really like to hear what members who have tested guarantees before a judge have found.
Tell us honestly, where have your guarantees worked and where have they failed?