Here is why you get blotchy staining on maple.
When the maple tree is growing it comes under stresses (wind, fighting for sunlight, growing on a hillside, etc.). It reacts to this stress by forming a special type of wood, called tension wood (hardwoods) and compression wood (softwoods). Both are called reaction wood; makes sense, as the tree is reacting to an outside force.
The tension wood cells will typically be more cellulose (cotton is 100% cellulose for example) and much less lignin (the stiffener and glue that holds the cells together). As a result, the tension wood cells are very weak (they like to fuzz rather than get cut off cleanly) and are very absorptive of liquids. This would not be too bad if the entire wood surface were all tension wood, but in fact, tension wood is scattered here and there. So, when staining tension wood and normal wood, we get two different absorbencies and a blotchy appearance.
My Solutions for tension wood… For staining and finishing, I use a propriety product to saturate these tension wood cells so stain will flow evenly across all the woods cells. I then seal the surface to stiffen the surface fibers so they can be sanded down rather than be pushed over forming more blotches.