I was in a choir for a short time, following an SATB score, and for the life of me I could not sing the tenor part on songs that I already knew. Try as I may I just followed the soprano melody line. On songs I didn't know however, if I learned the tenor part from the dots and made sure I never heard the main tune until I know my part I was OK! With me I think it is just conditioning.

I do find I can throw the odd singing harmony in when they are obvious and when I play concertina I play more than a few harmonies by accident:-)

Cheers

Dave