Do the model actually run through successfully when there is no log file?
Reason why I'm asking is that we experienced this as well and I found out that there are more documents reloading at the same time than what is allowed to run concurrently in QlikView Management Console. Meaning that the model does try to run through Management Console but because it reached this limit it doesn't actually run at all. You can see this in the task details under QlikView Management Console > Status > Tasks > "Your Model" > Show Task Details.
Where you can change this setting is here: System > Setup > QlikView Servers > "Your Server" > Performance > Max Concurrent Reloads.
Hope this helps.