Be sure to thoroughly check the wiring for issues. One issue I had once, was right where the wire comes out of the frame through the rubber grommet, is was a pretty tight bend and the wire eventually cracked and then worked intermittently. One I pulled that wire out a bit more, found the crack, fixed it with a splice, it worked fine. I replaced the wiring harness as soon as I could get one in. Do not want to have splices in the wires as that will drop the current down enough to not have the strength to pull the servo. I have had Mini Zinger wingers for several years, have 2 of the most modern version, have never replaced a servo. The Minis are strung tight, and if there is any drop in current they might not pull the servo. I suspect you have a wiring problem. I am sure you have done this too but be sure your electronics are fully charged. I had one receiver that would not throw one winger, but the other receivers would, I have Sportdog, they were in warranty and they replaced it, the replaced one worked fine.
waterdog, I would call Zinger, I have a few times and they were great to work with.