Having Trouble with Mixed Voice

Hello, I'm Resnard. I have troubles singing in mixed voice when it comes to vowels I, E, and U. Although I could easily sing mixed using vowels A and O (I could go all the way up to C5 using these vowels easily). My passagio is at F#4, and the song requires me to hit A4 in an U and E vowel which I have troubles hitting. I tried modifying the vowel to an AH and AE to compensate for it, but it isnt exactly the sound I am aiming for. How could I improve my mixed voice so that I could hit those notes in those vowels? Is there any particular exercise?
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