Morning Vimto, I love these trees but don't have the room. The non-flowering even at 20 yrs seems about right, and yes - should flower soon./
As it is in the Magnolia Family the amount of diseases they have would be very small, and considering that is hasn't been transplanted anywhere, the possibilities seem to be:
1) Aphids - apparently they love tulip tree's and my be the reason for the lack of foliage at the top
2) Chemicals - has anyone (neighbours) out anything different on their garden near the tree this year that may have got into the root system? Weed killer/ spilled oil/ petrol etc
3) overproduction of their own sap - do the upper branches have 'sooty' marks around the leaf buds? This would be a sign of it and would inhibit leaf development.
Whatever the cause, I wouldn't necessarily trim back (unless, of course you think it is some sort of mutation which might spread to nearby branches) and just see what happens next year. If one of the above signs is showing, you could try and treat the tree.
Hope it recovers soon, all the best
x