APM 2.8 PosHold mode problem

I cannot set the PosHold mode, when radio pwm is in range of the PosHold flight mode, the mission planner execute the mode which i’ve set prior to poshold,
although HDOP is below 2,and it found 9 sattelites! also the MP is telling my location accurately on the map!
Ive got a quadcopter with apm2.8, and AC3.1.5 installed, and ublox 6m as gps module(i use seperate compass 'coz the compass of gps module doesnt work well)
I would be so grateful if anyone here share any solution.

I would check and make sure your compass is setup correctly. By looking at the flight data screen and changing the direction of the vehicle and make sure all directions show up as correct. There may be a conflict between the two, which can do really bad things like fly aways.