KTM, we aren't fighting, just an enthusiastic exchange..
Everyone's suggestions are valid, but I look at the overall picture.. I was concerned that the result of the suggested 'full travel shake' would be a dumped fire. Seeing that I had done that very thing myself. I didn't want you to try it and end up more frustrated.. My 'don't do it' message was a bit strongly worded.. but I was concerned. So we had an 'enthusiastic exchange'. No harm done, And now there is additional info available that Capecoaler sumarized in the above post.
One other variable that will make a very big difference is the coal you are burning. Some coal burns down to a nice powder, about like talcum powder, but some coal burns down to a crunchy mass of burnt out 'husks'. These can melt together to make clinkers, and these clinkers can be very hard, like a mass of molten glass. These can jam between the fingers of the grates, and sometimes you cannot get them to 'unjamb'. the result is the fire has to be allowed to burn out, and you start over again with a freshly cleaned out firebox.. or some folks have forced the shaker handle and bent/broke the mechanism. Just be aware of how your coal burns.. if to a powder, you are in luck, if to crunchy masses, then you need to use your grates to grind up the ash, so it will be small enough to fall through the grates..
If, once you are done shaking, seeing hot coals/embers falling through the grates, if you then take a look at the underside of the grates,, you should see an evenly 'red glow' above the grates.. If you have some dark spots, it is probably ash that has not ground up and fallen through the grates.. Here most of us use a poker, usually a 1/4" or 3/8" round steel rod with a 3 or 4" 90* ell on the end, and a handle or loop on the other end.. use the 3-4" ell to poke up from the bottom, through the grate, between the fingers to dislodge the ash in the dark spots.. Also, this proceedure is often needed around the front/back and in the corners of the firebox.. as you get more time and experience with your firebox, you will be an expert with it's characteristics..