I have a Little Gem Magnolia and it's in a raised bed with a large percentage of compost. I did that because our soil is quite alkaline and rocky as all getout.

It likes a more acidic soil, so you might want to check your soil ph and adjust it accordingly. You may be iron deficient. In our area, there is plenty of iron in the soil, but I guess nitrogen or something can bind iron and make it unavailable to plants. So we spread ironite on our yard every spring.

Oh and by the way, the zones can vary quite a bit. A zone 8 on the west coast is not at all like the zone 8 here in Texas. I've found that plant sellers do not necessarily speak from experience when they say a plant will grow somewhere. Or you might have a micro-climate that has special needs.

If you need to amend the soil, I would suggest not disturbing the roots, but rather top dress.

Hope this helps.