"Why can I hum in tune but don't always sing the note I hear in my head?"

See tonyteach's post above. It sounds like you know what you want to do, your muscles just aren't doing it right.

There's a woman in my church who says "I don't sing," and she's a totally different kettle of fish. She doesn't seem handicapped, but when she tries to sing, she produces an erratic sequence of sounds which seem totally unrelated to the melody. There are big hops up and down, and no feeling that the song is in a certain key.

I wonder whether music teachers have any way to help a person with a problem like that.