In the past, I was able to View My Trips only if they had been booked online personally. Itenaries booked via corporate TAs were not available for viewing, though I could do online check-in, of course.
However, for my upcoming trip -- also booked through a corporate TA -- I'm able to use View My Trips. I'm not sure if there's been a difference in the way the flights were booked.
Loony