I recently experienced the same problem you had with the Four Queens silver strike machine, but at the Palms.

There were only a few strikes left in the Palms machine and red caps were the next to fall on two cylinders. Two other cylinders were completely empty. After playing for 10 minutes or so the machine stopped on an empty cylinder and the floor person said it had to be refilled.

The silver strike machines aren't being designed to count how many times stops are made on each cylinder. Normally you would think the odds would balance out or that the machines would detect which cylinders still have strikes and which are empty, but that is not the case!

The two remaining cylinders had 10 or more strikes remaining with the red caps much higher up. With the new fill I now had to play with all 6 cylinders and only two had a red cap close to the bottom.

When I started playing I thought the odds were a bit more in my favor since only 4 cylinders had strikes. Don't be fooled if you come across a silver strike machine with an empty cylinder. When that cylinder stops in the winning position the casino will do a refill of all the cylinders.

Jason