I am unable to access prior Time Machine backups.

I am unable to access prior Time Machine backups. I can see previous backups when I enter TM but they do not respond when I click on them. My TM is backing backups and at one time I was able to access them. I am logged on to my Mac Pro as the admin. I am running 10.9.5 and the TM disk is an internal 2 TB drive with 12 GB free.