When viewing a multi segment trip such as EWR-IAH-MSY, in the Manage Reservations - View Current Reservation screen, only the first flight of each trip shows a seat assignment. Examples:

Round Trip
1. EWR-IAH
2. IAH-MSY
then a later date return
3. MSY-IAH
4. IAH-EWR

Only segments 1 and 3 show seat assignments.

One Way (return after outbound has been flown)
1. MSY-IAH
2. IAH-EWR

Only segments 1 shows seat assignment.

If you select view/change seat, all seat assignments show and then if you click Continue and return to the Manage Reservations - View Current Reservation screen, all seat assignments show.