Why does opening the lift gate from inside sometimes not work?

When my CX-5 is running and in park, pressing the open rear gate button from inside...The gate opens, sometimes. Sometimes it will not open. Anyone?

Thanks.
 
Solution
It will not open if only the driver's side door is unlocked, you have to hit the door unlock button on the arm rest to unlock all doors before the hatch can be opened from the button on the dashboard.
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