Imprivata Radius for MFA, enrollment issue

We are implementing an Imprivata based MFA solution that uses radius. All works well until I add more than one server to the AAA pool for APM after that I have issues with users who are trying to go through the enrollment process. The enrollment process works by using Radius-Challenge messages and appears to break if APM spans the process across multiple servers. Is there any kind of persistance I can set up so that a session for a user is restricted to a single radius server?