Problem with pos hold mode

Hi, First sorry for my bad English …

I have a small problem with an apm 2.5 mounted on a heli kds450.
In pos hold mode, I can move it on all the axis except vertically,
when I reduce the throlle stick, the engine reduces its speed but the apm compensates
with the pitch to maintain the altitude .
so I can neither go down nor climb .
I need to set the stabilize mode to change altitude or to land.

I do not see a solution, if someone has an idea.
Thanks you very much.

And thanks to Google translate !!!

Please try to enable ESC GOV function, flight control is only responsible for control of pitch changes. If there is no ESC GOV, then the control ESC channel set to a constant value!

thank you for your reply. I have already tried to put a constant motor curve, it does the same thing.
The apm maintains the altitude with the pitch.
In fact, in poshold mode, I would like the pitch to vary depending on the stick and stablilises in altitude when the stick is in neutral. As for the right and left cyclic or rudder.
But I do not find this setting.