Assuming that you do know how it is supposed to work and you have the lever pointing at a tank (not in the middle), you have a defective regulator or a pretty drastic leak in the system.
With the two tanks open, and the valve pointing at either tank (working from two full tanks to start), the color dot will be green. As soon as the tank that the lever is pointing at is empty, the dot will turn red and start drawing from the other tank. At that point, the tank that the lever is pointing at should be empty, however, the other tank should still be full. You would then turn the lever to the tank that still has gas (the dot should then turn green), take the empty one and have it filled, hook it back up and turn its valve on. When the dot turns red again, the tank that was your "second" tank should be empty, the original tank will be full (you filled it a while ago) and you start the "exchange" process over again...turn the handle on the regulator to the full tank, take the empty off, fill it and re-attach, turn its valve on and wait for the dot to turn red again and repeat ad nauseum.
That's how it is supposed to work. Unless you are using an awful lot of LP and never checking the color dot (I make it a habit of checking it weekly), you should never run completely out of LP gas. If you are, and you are using the system properly, then the regulator itself is defective or you have a pretty good sized leak. The "system" is virtually fool-proof; there are those that don't trust it, but, a properly functioning auto change over regulator and regular monitoring will never let you down.
My 2 cents, your mileage may vary...
2 Cats; J-Lo and Ragamuffin :R
2014 Thor Tuscany 40RX DP
2011 Ram 2500 Longhorn CTD HO
2011 Keystone Cougar 318SAB (now gone)
2008 FunFinder X 210WBS (Sadly gone)