The type of straightforward roll over you are describing is deemed to be well tested and should work well. Enabling internal status printing can help with the diagnosis of this bug. In logback.xml, set the debug attribute to true in the configuration element, as shown below:

<configuration debug="true">
 ...
</configuration>

This will print out messages at midnight during roll over.

Here are a few questions that might be relevant.

1) Are your log files very large?
2A) Are there other processes accessing the log file?
2B) Is possible that more than one instance of your application is running?