The Time Machine status in OS X can be seen here in the system preferences. With messages from this process listed, you can see if the most recent activity involves 'deep traversal' or similar deep scans that may be the cause for the hangups.
You can usually see this by going to the system console, and searching for the process 'backupd' (the Time Machine background process). Therefore, it will perform a 'deep traversal' of your system, which may take a long time. If you have not backed up in a while, then Time Machine will not be able to create a quick incremental backup, as it will not be able to verify the existing backups match the state of your system.
There are several reasons why this may happen, including if you have not backed up in a while, your backup drive is full, problems with the current backup instance, and faults with the backup drive as a whole. While Time Machine should maintain hourly backups, there are times when it can stall and not progress beyond its status of 'Preparing backup.' You can see this status show in the Time Machine menu extra, as well as in the Time Machine system preferences. Apple's Time Machine is its built-in full-system backup technology, that will back up to networked storage like Time Capsule devices, in addition to locally attached hard drives.
