Magnolia Soulangeana

Hi for last 10 years my magnolia soulangeana has thrived with big dark leaves and nice big globlet looking flowers however last year the showing was really poor with really small looking globlet flowers and leaves that were really small too, this year so far there has been no sign of any flowers there are a few very small buds not many, overall the last two years inc this one dont seem healthy is there a problem with it i also have a magnolia susan and that the same it leavung me pulling me hair out can you advise please

thank you

graham