You really should not poke at an anthracite fire, but I have experienced what you describe. I believe you are talking about bridging, where the mass of coals does not settle after shaking and you need to knock the fire around to get the base of it to settle on the grate. Not sure what the issues are, but only experienced it when I first started to burn coal. I believe part of it is caused because of the taper in some fireboxes is a little to extreme. It could be from overfiring perhaps, are you getting a lot of fused ash?