Roles and permissions

Hi,

Since I upgraded my Moodle installation from 1.6 to 1.9, I cannot permanently assign teachers to the courses. I go to course, roles, I assign the teacher to the course, but:
1) sometimes I cannot set the duration of the role to "permanent" (I do not know if this is the right word: I am using the Italian version): it authomaticallì resets to "30 days" or something like that.
2) sometimes I can indeed set the teacher as "permanent", but after a (for me) random number of days, the teacher is unenrolled from the course, and the course results to be without teachers. Of course this is really ennoying, because I must continuouscy re-assign the same teachers.
3) The platform is set to automatically unenroll students after 14 days without connection. Could be this affecting teachers too?

Thank you.

Massimo Manca