Magnolias prefer rich, moist and preferably acidic ground to grow in - it may be that the ph isn't right, or the plant just doesn't get enough water at times.

If the soil is currently nice and moist, water on some Sequestrene iron tonic, scrape up the top of the soil round the base of the tree a little bit (gently, magnolias don't like root disturbance) and sprinkle 3 or 4 good handfuls of Growmore granules, then apply a mulch over the top. If its not moist, it needs a good soaking before you do anything else, if that can be managed. Pine bark or pine needles make a nice acid producing mulch, but peat does this too. I'd suggest you mix peat with organic soil conditioning compost or your own garden compost and bark chips over the top, so it doesn't just blow away when its dry. This mix will be fairly acidic and the compost element will help to enrich the soil. Apply the mulch around the base, about 2 or 3 inches deep, out to the edge of the leaf canopy, but leave an inch clear all round the trunk. Hopefully, this treatment will help.

The other thing to do is check the trunk and branches for signs of scale infestation or any other infestation/disease.

Sorry Bamboo I went to the one below, I couldn't really help.

That's absolutely fine Pamg - you're fully entitled to do whatever you like and certainly don't owe me an apology;-))

I'm pleased youcould advise more than me :0)