mongodb is not starting up properly ubuntu

On occasion mongoDB doesn't start.

When I attempt to start service manually ...

$ sudo service mongodb start
mongodb start/running, process 10243

It appears to start but still doesn't ...

$ sudo service mongodb status
mongodb stop/waiting

Why is this happening?

EDIT mongodb.log

Thu Sep 15 20:01:22 MongoDB starting : pid=10559 port=27017 dbpath=/var/lib/mongodb 64-bit 
Thu Sep 15 20:01:22 db version v1.6.3, pdfile version 4.5
Thu Sep 15 20:01:22 git version: nogitversion
Thu Sep 15 20:01:22 sys info: Linux allspice 2.6.24-28-server #1 SMP Wed Aug 18 21:17:51 UTC 2010 x86_64 BOOST_LIB_VERSION=1_42
************** 
old lock file: /var/lib/mongodb/mongod.lock.  probably means unclean shutdown
recommend removing file and running --repair
see: http://dochub.mongodb.org/core/repair for more information
*************
Thu Sep 15 20:01:22 exception in initAndListen std::exception: old lock file, terminating
Thu Sep 15 20:01:22 dbexit:

Thu Sep 15 20:01:22 shutdown: going to close listening sockets...
Thu Sep 15 20:01:22 shutdown: going to flush oplog...
Thu Sep 15 20:01:22 shutdown: going to close sockets...
Thu Sep 15 20:01:22 shutdown: waiting for fs preallocator...
Thu Sep 15 20:01:22 shutdown: closing all files...
Thu Sep 15 20:01:22     closeAllFiles() finished

Thu Sep 15 20:01:22 dbexit: really exiting now