"last" not showing recent login activity

[Matthias Petermann]

Hello,

on one of my systems I just found out that "last" only shows some old  
login / logout activity, but not the recent actvities.

The strange thing... everytime I log into the system, /var/log/utx.log  
gets update to the current timestamp (and also grows by some bytes).

But "last" only shows very old data...

srv# last -f utx.log -d 20121218
matthias   pts/3                           Mon Dec  3 23:32   still logged in
matthias   pts/2                           Mon Dec  3 23:31   still logged in

Is there any reason why I can't see the recent logins there? Which  
component does write data to utx.log - is this done via syslog or a  
lower level mechanism?

Thanks,
Matthias