When functional harmony doesn't answer the question, there are other ways to figure out the key. #1 learn the melody. I have a very cool lesson coming up for my video series that I shot Friday, focusing on exactly that. Chord progressions, alone, can be tricky, there are a number of things one can look at to discern the tonal center. BTW, not enough bass players learn to play the melodies of the songs they play
P.S. I like your lessons/book. A lot of valuable information.
Yes, chord progressions can be tricky, and there are a lot of reasons why.
Some chord progressions are written by gifted but not quite harmony/theory-skilled musicians.
Sometimes, a lot of chord substitutes and "passing or added chromatic chords could obscure the tonal center.
Also, without knowing about Modes it's kind of difficult to learn about Modal Interchange.