I agree with Suzuki27, The length of the entire "indent" mechanism which includes the plug and gasket, the spring and the cone shaped slide all need to be correct, also make sure you fit the spring into the plug first then the slide over the spring before screwing the plug back into the engine casing.
I once pulled a RM125C motor apart to find some dope had gone and done an oil change and when refitting the plug, put the indent slide in upside down then the spring into the hole in the back end of the slide. The end result was the spring ended up being tangled around and through the holes in the selector drum and stretched out to about 75mm long. He probably then wondered why the thing didn't change gears properly to start off with, then got worse as the spring became more tangled. Becoming fed up with it, he decided to scrape the motor. Thankfully it didn't get chomped up in the gears and cause any serious damage.