Serial monitor works, serial.print() not

Hello

I'm trying to send NMEA message from Adruino uno to navigation instrument. When I use Serial Monitor to send message, it works fine. Sending the exactly same message from adruino application serial.print() does not work. The Serial monitor still monitors the message but the instrument does not get it.

Please help (this is my first adruino project)

Are you sure you are calculating the checksum correctly? Did you end the sentence with a '/r/n'?