Forms Authentication problem across subdomains

Hello,
 I am having a strange problem with a ASP .Net 2.0 web application while trying to maintain authentication across subdomains.
In my web.config, I have forms authentication enables using cookies in which I have the domain property set. When I'm going across different subdomains everything works perfectly fine.
structure of my domain , subdomains is like this:-
abc.com (That is creating authetication with authetication cookie)
a.abc.com (That is using authetication of the domain with authetication cookie)
b.abc.com (That is using authetication of the domain with authetication cookie)
In one of my subdomain server i could not find the authetication cookie, while on the other one i find authetication cookie and the identity.IsAutheticatd returns true.
I could not figure out ther eason why it is not working on one server and is working fine on another server at the same time. To figure out the problem i ...