Southern Magnolia normally sheds some old leaves in spring, so some of its appearance is normal, but if it isn't receiving lots of direct sun any more due to nearby trees, that is likely a main factor behind poor flowering. Drought stress can also affect blooming the following year, and much of Maryland experienced drought conditions for most of last year's growing season. Was the tree irrigated periodically? If not, early-flowering trees like Magnolia that are stressed by drought can either not form flower buds (since they develop them months before they open) or may form them but they might die off long before when they would open. If you think it's in need of water, see what recent rains did by feeling the soil about six inches deep within it's root zone. If somewhat dry to the touch at that depth, watering may be needed. If damp, the tree probably has enough moisture. Magnolia has relatively shallow roots compared to other trees, so is more vulnerable to soil compaction and drier conditions.

Is there any bark/trunk damage visible? We don't see any in the photo, but it's hard to see the trunk clearly. If no sawdust, holes, cracking bark, oozing sap, or other abnormalities are present, then there probably isn't a pest or disease issue at work. (Whatever minor fungal leaf spot may have appeared on the shedding or older leaves is inconsequential and does not require treatment.)

For now, the tree doesn't appear to be in great distress, and watering (as-needed) may be all it requires. If decline becomes more prominent or symptoms change, you may want to consult with a certified arborist who can examine the tree in person, in case there is an issue we can't see or a problem with the roots that is obscured by soil, like girdling roots. Arborists can't always fix every problem, but they can help to identify sources of tree stress that might be able to be alleviated to help a tree recover (if/when recovery is possible). Our general impression from the photos is that the Magnolia isn't in great danger of decline, but someone on-site may have a more informed assessment.

Miri

It's hard to tell what the cause of bark flaking is...for some tree species, this can be a normal trait for mature trunks, but in this case it does look excessive. Some of the old branch stubs, even though they are gradually sealing-over, look like they have wood decay in the heartwood. This is not necessarily life-threatening for the tree (the living layer of a trunk is just under the bark, as heartwood is naturally dead wood by that point), but if it were to spread or worsen, might cause problems with tree stability over the long term. Trees do have ways of compartmentalizing decay internally, though, so if those protections are in place, there won't be a way to detect them from outside of the tree. (Nor is there anything that can be done at this point...no treatment halts wood decay, and pruning wounds should not be sealed with anything.)

Tree climbing, while of course enjoyable for children, is not recommended to protect the health of the tree. Bark removal can pose serious health risks to a tree which will not be treatable, and while some tree species have much thicker bark layers than others, there is a limit to how much bark (itself a non-living layer) the tree can replace once it is lost. We suggest discouraging climbing from this point forward to reduce the risk of trunk infections or damage and to minimize any weight strain on lower limbs.

Miri