If you're having difficulties allocating existing clients to the attendance roll its possibly due to the Expression of Interest booking type.
When a client is booked under Expression of Interest, a ticket package hasn't been added
1. Go to the Activity Summary, Request tab
2. Go to Actions on the Client Tile and select Update Status select Action Required
3. In Actions select Change details in the Booking Type change from Expression of Interest to Booking then select a Ticket package
4. In Actions select Allocate Sessions