This probably isn't your problem, but I have it occasionally. On my 401, I miss putting the needle in the right hole. There is something in the same area (Haven't gotten a mirror to look at it) that feels like you're putting the needle in, but it just falls out. It's right behind where the needle goes, so on the 401, it's easy for me to miss. I've done it so many times now, I know what the problem is and just try again.
If it were mine, I'd get a mirror and a flashlight and examine it carefully to see what is happening as you tighten the set screw. You might even have to use a magnifying mirror to be able to see what's happening.
Other than that, I have no ideas.
bkay