Help````Can`t change flightmode

Hi guys,
I get a problem,My quadcopter can`t change flightmode,when i takeoff with Althold mode,it was ok,but change mode to loiter fail.But just few second later(20s),it will be ok.
so , any body know about this what does it happen?

You’re not getting good enough GPS signal before you take off. HDOP of Less than 1.0 is what you want.

The easy way to check is switch to loiter while on the ground and see if the LED is flashing green. If it goes yellow then wait till it goes green, them take off in any mode you like.

Thank you for your answer.