Mission planner refuses to write waypoints loaded from a WP file

I run the latest stable version of the mission planner and use AP3.8. The plane refuses to accept ‘write WP’ commander even though the Home position is set and locked.
The plane accepts ‘write WP’ Command when i make a new flight plan and use ‘write WP’.
I use a Windows 7 professional computer.
Any input is much appreciated.