This is a long-standing problem. I have found that the only way to deal with it is to have the film screenings in class (I tend to lecture over parts of the films anyway) and try whenever possible to be given extended class time. I have also occasionally made watching a particular film part of an assignment, making the films available on reserve at the library, etc.  
I'va also tried scheduling screenings of supplementary films during non-class times, but very few students -only one, in fact - actually showed up.
Robert Hunt

----
Online resources for film/TV studies may be found at ScreenSite
http://www.ScreenSite.org