star,
the only trouble I have ever had booking on nwa.com (and i've booked nearly a hundred flights) is when I try to build a flight with multi cities and I choose the times.
Even if they appear to work, connection-wise, an error comes back and says there are no available flights.
the last time I did this, I called and got the flights I wanted and later figured out what I had done wrong.
Perhaps you experienced a similar glitch.
I would have understood what I did wrong if the error code was more accurate like:
try different times, these don't work, dummy!
The nwa.com web site is the best! I find it easy to make reservations and easy to navigate in general. Don't give up.