Unfortunately the links to the Photography Guide did break again so just visit http://photosynth.net/howtosynth.aspx to pick up the guide as well as the video introduction.
Picking back up where I left off... Because the two shots of the harvester machines were so different (I'm not sure that I can even find the same trees in the background between the two), Photosynth couldn't link them and the shots just weren't close enough to the harvesters to pick up any clear details. Also, remember that I said Photosynth is looking for stationary things. In this case, only two shots of the harvesters in the field isn't enough for Photosynth to decide whether it's meant to be tracking the harvesters or the field. (I assume the harvesters were in motion with all the dust in the air.) Photosynth could probably link those photos if you had just taken more pictures between the two.