Shows get moved when new show are added and unfortunately there no timetable when a particular show get moved and it usually depends on when new show get added.
The annoying thing about this is that some months there hardly any new show added than all of a sudden new show get added weekly.
The best advice is to keep track on when a new show you like get added-don't delay
don't tell yourself I will watch it later if it's free and added watch it as fast as you can while it's free
. That what i do.
if your busy than allocate 3-4 hours on a weekend- hell if you love a show wake up 6am watch it