Time Machine troubleshooting
Why do I have an explanation point within my Time Machie desktop icon? What is the problem?
MacBook Air, OS X Mountain Lion (10.8.5)
The exclaimation point is telling you, for some reason, your back up has been delayed. If you right click on the TM icon you should see this.
Try clicking on Backup Now (right click on TM icon), keep that contextual menu open and watch to see what might be happening. Most of my issues for this is due to the back up drive not being available.