Heater might not be working right, but before buying a new one, try this. This should basically reset your heater.
I see you have it cranked all the way up.
1. Unplug your heater.
2. Then, dial it back down so that it is at the bottom of the temp range.
3. Let it sit in the tank for about 30 minutes.
4. Plug the heater back in.
a. Now, let it sit about 10 minutes.
b. Now turn the temp up 1 click at a time.
c. Stop when the light comes on.
Now you can turn it up a little at a time until it's maintaining the temperature you would like. You probably won't have to reach the top of the heater's range.