I quote from the movie "My cousin Vinny"

It's a procedure. Like rebuilding a carburetor has a procedure. You know, when you rebuild a carburetor, the first thing you do is you take the carburetor off the manifold? Supposing you skip the first step, and while you're replacing one of the jets, you accidentally drop the jet, it goes down the carburetor, rolls along the manifold, and goes into the head. You're f....d. You just learned the hard way that you gotta remove the carburetor first, right? So that's all that happened to me today. I learned the hard way. Actually, it was a good learning experience for me.

I always wedge rag in the openings, but make sure it is all out before you turn the motor over, but use a magnet to remove the shims and a magnet to replace, but they tend to twist on the magnet, so I use a pointed scriber to locate them.
If they do not want to go in, then dont persist, but pull back and realign them and try again.

Dave.

Dropped some doing my valve clearances. Got a magnet on what looks like to be a car aerial. just fished around with that, eventually got it out.