In the last few weeks we had seen a couple of posts where people were unable to read from the analog sensors and some other sensors too but were able to run the LED blink example.
We have found the problem with the board and it was because of the fuses being set incorrectly on the firmware which reduced the clock speed to 1/8 the of the original and caused the problem. There is a script attached to this post and running that on your pi should make the problem go away. To run the script, make it executable:
sudo chmod +x grove_setup2.txt and run it:
Do let us know if this works for you.