What does the switch say if you run 'show auth sess int gi a/b/c det' at various points during the pre/during/post authentication?
If you get to a successful post-auth state quickly, then it's unlikely to be an ISE issue and you want to be looking at the Switch and the Client.
What switch hw and sw are you running? Can you share the Show Run output?
What is the Client device? OS? What version of AnyConnect?
Is this a new issue following an upgrade of something, or a first-time install? If the former, what did you upgrade?