Yea, that tank is only 4" deep, so sensors aren't super accurate especially if it's not level. I have to raise the left like others have said to get mine fully empty. I look stupid as hell doing it, but it works on my hoopty.
I second the dish soap idea every once and a while to keep the film off the inside of the tank. I use a little dish washer soap, plain dish soap will work too. The nice thing about the dish washer soap or Tide, it doesn't suds up as much.