There are several reasons why one can sing off pitch (off key), or more or less flat. It is sometimes normal to have a few glitchy notes when you are in your learning mode, but there are three main reasons why you might be singing flat.
LAZY EAR
Having a lazy ear usually means you do not have enough experience in music. Not having a musical background is not much of an issue here, because you can learn things at any moment of your life. The mind and the ear do not communicate well in order to determine the right pitch, which makes it harder to sing the right notes. But not to worry, you can develop your ear training by learning music, notes, and by having a voice coach by your side. Your voice teacher will correct you and place you back in the right direction. By patience and keep practicing!
Watch this 2 part video that will teach you how to Practice:
How To Practice – Part 1
How To Practice – Part 2
TENSION
When tension settles in, the voice tends to have a hard time coping with this trauma. Therefore, the pitch is jeopordized. When you are off pitch or flat, it could mean that you are pushing the sound out instead of focusing on the air and on easy singing. Remember that singing is all about flexibility and ease, and everything that is an alien to the body will only harm it.
There are several ways to can get rid of Tension:
Remove Tension
Breath & Legato
How To Sing Legato
SOFT PALATE
I see the soft palate as a filter and a way to transition from one register to the other. Once this soft palate is not lifted, this could cause the throat to tense, and the pitch to become flat. Maintaining the “Inalare la voce” position is crucial when you are singing in order to maintain the form of the inhalation while producing sound.
So make sure to watch the video about The Soft Palate