Colin, it would be interesting to hear your reasons for not completing them.
In the case of Introduction to Genetics and Evolution, my computer became infected with a virus in the week that I had to sit the final assessment and submit my final assignment, so I was unable to complete the course, because I spent most of that week disinfecting my computer and re-installing software etc. I will repeat this from scratch, in part to remind me of what I have forgotten since the beginning of this year and in part because it is interesting.
For Logic and Language, I just didn't have the time to do all of the reading and assignments so I dropped out but I will start again from the beginning because I had not completed anything in this course.
I have signed up to take Emergence of Life again, which is in its 2nd week of lectures. I didn't pass this course the 1st time because I did not submit the assignments. I am also starting this from scratch because I want to complete it properly this time. It's an interesting course (to me) and I have come to terms with the "let's pretend" nature of a couple of the assignments, although I still do not like them.
I have sometimes withdrawn from a course because I just didn't like the subject or the teaching and assessment format and I was more interested in other courses that I was doing concurrently. Regards, Colin McLaren