Pre-booked seats not always available

In an earlier post I indicated how important I found being able to select convenient and comfortable seats, in advance. However, on a couple of recent NWA flights, I found that reserving in advance does not guarantee that you will receive the selected seat. In one case, I understand that equipment change was the reason. I had been moved from one exit row to a different row. The gate agent said a similar seat in a different row. This was actually not true, as it was the exit row at a main door, and therefore the legroom is curtailed by the slide housing on the door. But worse is the fact that this row does not have under seat storage, so my computer bag would normally not be within reach during the flight. As I like to access my computer to complete expense reports, and also my Shure noise reduction headphones during the flight this could have been a problem. Luckily the passenger next to me, who had unused under seat storage allowed me to use his space.
On flights a week earlier, my preassigned seats had been taken away on both legs. In this case there was no obvious equipment change so I wonder why my assignments were lost. This must have been a computer glitch, as the chances of someone traveling the same two segments and arranging to 'take over' my seats on both legs is quite small. As I had not checked in online for these flights I thought that perhaps it was due to the airport check-in, just 60 minutes before departure. However, the experience mentioned above indicates that even early check in does not guarantee the seats.
Sorry NWA but another soduk for you.