so, explain in a little more detail what a "mag spot" is.  Is this a place where someone removed topsoil?  Is this a place that was low, and topsoil was added to make it level?  or both?

The thought crossed my mind that when you are pulling soil samples, you are pulling from the filled soil.  However, your problem may be deeper, in the original soil - especially if this is a filled spot. 

Maybe there is value in getting a backhoe this fall, digging a hole/ trench in one of these areas, and seeing what the soil profile looks like down thru.  Maybe get someone with a soils background to help you understand what you are looking at, if you don't have that kind of background.  That's the first thing that crossed my mind.