single-note dynamics in sound fonts?

I wrote a diminuendo over one long note from f to p. On playback it did not play it. The sound was Roly's trombone. If I switch to trombone expr it does work. Is there something with certain samples, that do not support single-note dynamics? (is that the whole soundfont or just Roly's trombone?) Is there a way to make this soundfont react to single-note dynamics?