Schubert
After Beethoven exploring so many possibilities to deal with sonata form, most composers had to think of a real masterwork to entitle it a Piano Sonata (unlike for example with Haydn, who could write even more than 50 sonatas). If you look at Liszt, Brahms or Schumann, they all were honouring Beethoven and considered themselves his heirs. It is the same with symphonies (compare over 100 symph. by Haydn to ridiculous 9 of Beethoven

).
Schubert, though, lived nearly at the same time as Beethoven, so he probably does not count. Prokofiev and Scriabin, although kind of connected with the musical past, have revived the Sonata in a new way, by connecting it with modern ideas etc.
Also take into consideration that during the Romantic Period, composer were discovering new ways of expressing their ideas, ways in which they would not be bound to strict rules, although these rules have been bent quite far. These rules seem to have made the sonata improper for most ideas of the great composers (so, it is easier to say, well, that is a nocturne, than to try to (ex)press the same ideas in a sonata movement), whereas some (Prok./Sriabin) were able to give this form a new meaning in their own style.