#1
okay so I've recently conquered my fear and started singing a few of the songs I play.

I've noticed this strangeness in that I really struggle with some songs while others seem to be easy. Now that sounds obvious, but the thing is, the songs I screw up seem no different to me than the ones I can do okay.

For example,
when I try to sing black balloon by the goo goo dolls (which is in the key Db maj), I fail miserably. Go way out of key even on the middle range notes.

Then, when I do sympathy (key of G# maj), I seem to do okay.
Same goes for "if you could only see" by tonic which is Bb minor (which is same notes as black balloon's key of Db major)

what am I missing here?
this is a post. there are many like it but this one is mine

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#2
maybe the backing chords are throwing you off?
I remember we were doing smooth operator and the singer found it hard to sing the ninth of the chord we were playing..

or maybe it is just your range?