Number of sats is not the indication of a suitable lock.
The HDOP has to be low and be consistent enough that the EKF is happy (search the Arducopter web site for references to these)
If the device has been shipped a long way you may need to wait until the GPS downloads a new almanac.
How long have you let it sit and waited for the lock?
I have know it to take 20 minutes sometimes.
Make sure you have clear sky above the GPS