thank you for yr reply!
when i perform a soft reset, there is an error message "Sorry! Bluetooth can no loonger be used because the license is expired". but my license key is still valid.
as u suggested, i am now using channel number 12, when i advertise my service, it runs normally (both x50v and x51). when i attempt to create a stream via my service, it works only x51 -> x50v. for x50v -> 51, it throws "Argument out of range exception", "null reference exception". actually, two devices are running the same application, but why response so differnet?