You've had the Lyman 55 for a while, so I doubt you're doing what you might possibly be doing. But...you have the 3 sliding brass pieces (large, medium, and small) that you adjust outward (screw adjusted) to your right the necessary amount to open up the cavity for measuring the desired amount of powder. If the small and/or medium brass slides are not open as far or farther than the large slide, you will get some really messed up powder charges because the powder won't all drop - being partially blocked or held back by the projection of the small and/or medium slides into the metering opening when you work the lever and the opening faces downward to drop the powder. Whew! I know what I'm trying to say, but putting it in words is difficult.
With the powder measure empty, just look down into the powder measure cavity. The most open slide should be the smallest (the topmost one) and the most closed slide should be the largest (the bottom one). Fiddle with the measure for a while and you'll see what I'm talking about. Early on in my ownership of my Lyman 55, I managed to mess that up and get some crazy inaccurate throws. Took me a while to figure out what I had done wrong.