why NxScene::fetchResults always return 0 ?

why NxScene::fetchResults always return 0 ? and the value of error is 1

NxU32 error;
UBOOL bOk = NovodexScene->fetchResults(NX_RIGID_BODY_FINISHED, FALSE, &error);

my game is build on ios10.3, and pyhsx2.8.4(built on ios sdk 8.1)