Well i would suspect there are no errors or warnings to log
You can check the log file with fuser or lsof and determine which process is keeping the file open.
Other then that, you will have to login to your app/db/system and determine what is going on and how it is configured to log.
There could be various reasons such as :
Missing disk space.
Files removed with rm while process is attached (you will not see the file anymore, but the process is still writing in it and taking up space until you restart it).
Inode exhaustion (rare on modern filesystems, limits are quite high).
Wrong files/log cleanup (badly configured logrotate and such).
etc.
What is 'expected log update?'. Can you elaborate on this ?