Did you replace that extension spring? Yours looks stretched. I had the same issue until I replaced mine. Found a new one online 10-320. There is also a compression spring that pushes the cam inwards. I suppose it's possible that if that were weak the cam might move away from the post, guessing since your rubberband fixed it.

I will admit it was a PIA to get mine done, but I let it cycle for 2 min in the coil test with not one failure. The other issue I ran into was the type of grease for the cam on the shaft. Since the pin that rides in the cam pushes the cam outward when the plug goes up, the cam needs to slide out easily, which in this design is tough because it gets pushed at an angle since there is only pressure from one side and can cause it to bind on that shaft. I tried three or four before settling on white lithium grease.

I think your rubber band is helping because when the pin pushed the cam outward on its way up, there is equal pressure on the cam from the band and the spring on the shaft up top that holds the cam against the pin. Without it I bet it was binding on the shaft and not allowing the plug to pop up.

If it works, go with it!

I recently did battle with the same mechanism on a big guns. The spring has to be PERFECT for it to function correctly. Took lots and lots of tweaking of the tension.

IMHO, it's not so much that spring, but the cam has to be able to slide out on the shaft when pushed on at an angle which is tough if the hole in the cam is worn and/or not greased properly. I couldn't get it to work at all with some types of lubricant. You should be able to grab the cam about halfway down and pull it out without it binding on the shaft. Grabbing from the top around the shaft doesn't apply pressure the same way that pin does on the lifter.

Rubber band will wear out very soon. Not a long term fix.

I had two of the mech and both would not function unless the spring was perfect.

So that is why i have a zip tie on mine. I guess its time to adjust it and replace the zip tie. I picked up my williams Fire! last night and the fire plug stopped popping down after a few games.