Recently a teacher to initially asked me to sub long-term for her this November (she asked back in March) changed her mind two weeks before I was supposed to start in favor of another substitute. Given how far in advance she asked, I was very confident I would be able to pull this off.
I had originally met her while subbing for another teacher in the room next door. During that teacher’s prep period, I would sit in on the teacher next door to watch how she ran the class. I did this for two weeks. Near the end of that assignment, she had asked me to be her long-term sub. Since it was so far in advance, I kept in touch with her, hanging out in the teacher’s lounge when I happened to sub in that school. From last March up until this week, I had clearly established to her that I was comitted to subbing her class. I had also told her I made arrangements to ensure I wouldn’t have any other obligations that would conflict (school, other job, etc).
Part of the reason I felt so frustrated with getting turned down now is because I had worked so hard to establish to her that I was a reliable and enthusastic person. I know that many teachers are kind of particular when they need a long-term sub; some teachers have a qualified friend/relative do it, others ask a retired colleague. One reason I work so hard to make a good impression (even if it is subbing for only one day) is the hopes that the teacher would call back/specifically request me, but in a year of substitute teaching that has not happened 
Granted this situation isn’t because I’m not getting called in to sub enough. Rather, it is a desire to sub long-term, and having an opportunity to actually help make the lesson plan, follow it, and grade papers.