Do you sing one word on a specific note easily, but another word on the same pitch seems much harder? You could probably use some practice and training on unifying your vowels. The ability to unify your vowels and make them sound as if they come from one instrument, having about the same high and low frequencies and blended with no cracks or breaks is one skill that separates the accomplished singer from an amateur.
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Jaw tension can also cause issues with vowel unification. Jaw Tension and Singers.