Some leaf yellowing/leaf drop in broadleaf evergreens is normal at this time of year, as the trees shed some of their older leaves. https://extension.umd.edu/hgic/normal-shedding-older-leaves-trees-and-shrubs Many broadleaf evergreens also had winter damage this year, resulting in some leaf loss. https://extension.umd.edu/hgic/winter-damage-trees-and-shrubs In your case, it looks like the trees also are located in too much shade, resulting in the thin growth and lack of flowering. These magnolia trees do not like to compete with other plants and they do not like to be in full shade. This is probably the reason for the gradual decline.
Thanks! That all makes sense. Do you think my soil also is a culprit? I mean is there anything I can do? Will it be fine?
Wait a few more weeks yet for new growth to develop. Again, some yellowing and leaf drop in evergreens is normal at this time of year. These trees need to get about 6 hours of full sunlight at least. You might consider pruning some of the trees to allow for more light to come through.