Have you ever noticed that certain adjacent notes in your range differ noticeably in volume? For instance, are your Bbs strong and your Bs weak? If so, you may be a singer with 'trick notes'. These volume imbalances can result if your vocal tract aka your resonator (throat, mouth and nose) resonate certain notes too well which can make adjacent notes weaker by comparison.
To figure out if 'trick notes' are a problem, you first have to find what your 'head' is; that means which resonant tone your vocal tract amplifies the best. Ladies, start by humming with teeth apart, lips together on an A below middle C. Keep moving the pitch up by half-steps until you get to a middle C#. If one of those 5 notes vibrates your whole face evenly AND is quite a bit louder than any of the other notes, you may have a trick note problem. Gents, try this around C below middle C up to F below middle C.
Personally, I've got a Bb head and strongly so. As a result my Bs tend to be weaker and thinner sounding. To make all my Bs better, I lower my larynx so that my Bs match my naturally good Bbs.
If you have a vocal tract which is strongly tuned to one of these notes, you may hear a weakness on the notes which are a half-step higher or a #4 higher throughout your range.
Which 'head' do you have?