I'm assuming you are talking about the #494 Rotary Beacon Tower.
Possibilities:
1: Needs new drive washer.
2: Voltage is too high or too low. They can be very finicky about voltage, and it's best to connect them to a small separate transformer that can be set to just the right power level. The fixed-voltage accessory posts of most transformers usually don't give the degree of fine control needed.
3: Clean the circular pad on which the driver washer fingers run. I use a Q-Tip.
4: Dust a little powdered graphite on the pad.
My #494 has been running continuously (barring a couple of power outages) for about five years without problems. The above suggestions are what's kept it that way.
Hope this helps.