Here is a list of things to check when you start shearing pins. Never replace shear pins with a nail or other type of pin. A shear pin is designed to break at 25 foot pounds. The large cotter pins break at 65 foot pounds.
Even when using all three worms, with a total length of 11 feet, it should be relatively effortless to hand crank coal in the pipe.
Is the end of the worm exiting the pipe into your bin more than about 8 inches? It should be about 1 1/2 spirals of worm into the coal supply bin and this range can vary from about 3 inches to as much as 10 inches, with mid-range being preferable and can even be more spirals than 1 1/2. A new worm and pipe will have the worm extending past the end of the pipe about 8-10 inches. If you would cut the pipe to remove a corroded end, the worm extended length will be longer but may work just fine. Some owners will cut the length of the worm, rather than preferably buy a new one, and this would be done at a weld in the spiral. Too long a section of worm into the bin will grind up the coal, causing packing.
There could be too much coal on top of the worm. Keeping 12-18 inches of coal above the worm helps reduce the chance of carbon monoxide from escaping the bin and entering the house, but too much coal above the worm causes grinding and fines that combined with wet coal will cause packing inside the pipe.
Are the bin feed pipe and burner end pipe tight against each other in the pipe coupler? If they are not butted together tightly, coal fines will start to fill any gap and start grinding up the coal as it passes by. The steel pipe can be rusted with pinholes inside the coupling or anywhere along its length. This will also create a packed zone of fine coal that binds up the worm. Split couplings are available instead of the standard pipe coupling. They are two halves which are bolted together.
Are you getting a good supply of rice coal with mostly uniform size? Many people are using buckwheat coal instead of rice coal to save money. It puts more stress on the worm feed as does alot of shale.
Wet coal will also cause the fines to lump together, adding stress to the worm.
Have you checked the condition of the worm? The burner end worm is the most common to be replaced. The edges of the spiral should be flat, where they would contact with the inside diameter of the pipe. If the edges are sharp, you will start grinding the coal inside the pipe and cause packing. Check especially at the bottom of the pot, the last few inches of the worm.