China refuses to sit down with everyone involved at once... they insist that the only way they will talk is with one nation at a time, and that all resulting treaties must be between themselves and each other nation individually.
Naturally, the other nations insist that the disputes must be solved "all-at-once" so that there are no contradictory treaties, and so that they can stand together against China.
The UN put out a proposal over a decade ago that assigns all but the very center group of islands to specific nations, but no one has yet accepted that, since it didn't completely solve the issue.