No Position Information below EKF2 using RTK

when I am using a RTK below EKF2, QGC display “RTK 3D lock”.But the x,y informantion in LOCAL_POSITION_NED in mavlink inspector are both zero,and it cant be switched to POSCTRL.
However, when I replace EKF2 with INAV, my vehicle can detect the GPS and can be switched to POSCTRL.
Do I need to tune some parameters?
Need some helps,Thanks!