The problem is inadequate ignition, which can be caused by several things. jnichol6600 is probably right about a hotter primer being the cure, but there are other factors that may have contributed to the problem. Using a plated bullet might be part of it, the core is very soft lead, and the plating is not as stout as a standard bullet jacket. That means it takes less pressure to move the slug out of the case neck, and if it moves too soon, pressure doesn't build to the proper level for good, consistent ignition. The powder charge fizzles and fuses into a solid mass, as you saw. A firm crimp, a more stoutly-constructed bullet, and a hotter primer will all help to assure you get good shotstart pressure, and a complete powder burn.
Other potential causes include a weak primer or some type of contamination in the case, which makes the powder harder to ignite.
I don't use plated bullets in magnum-class loads, I'm convinced they are too soft and too easily deformed, ruining accuracy. They are designed for softer-shooting loads, particularly pistol loads at target velocities. Some also caution that a firm crimp can also cut into the plating, and a stuck jacket in the bore is an accident waiting to happen.
Stick with hard-cast or jacketed bullets in magnum loads, and save the plated stuff for target loads. 8)
