Remember that but needs an independent subject and verb after it in order to have a comma in front of it.
...didn't really see him, but she tried to get him on the phone.
...didn't really see him but tried to get him on the phone.
...saw the car coming toward me, but I really thought he would stop.
...saw the car coming toward me but really thought he would stop.
...saw Marianne, not her sister...
...saw Marianne but not her sister...
...went in on Mondays, never Tuesdays...
...went in on Mondays but never Tuesdays...
Happy punctuating!
Margie