time machine not backing up.

My time machine looks like it's backing up, even when it's not plugged in. But when I look at the back-ups, it actually just shows me my current hard drive, they aren't historical copies at all. It seems to have started a month or so ago. All back ups up until August were vallid back-ups. When I click on one of the back ups say from last week, they are not back-ups, they just show my current desktop...Can anyone help diagnose this please? There's plenty of space on my hard drive.